May 1960 - July 1971 - Thorpe and Porter
On the 4th November 1959, the British government announced the end of import restrictions on cheap books and magazines from the USA, in place since the end of World War II. Following this, Thorpe and Porter (T&P) of Leicester began officially distributing US Marvel Comics in the UK. The first batch of books imported by T&P were cover dated May and June 1960 in the US. The last batch were cover dated July 1971, after which the distribution contract was taken over by World Distributors.
For books cover dated May 1960 to the end of 1964, and August 1965 to November 1967, Eastern Color Printing of Waterbury, Connecticut printed specific UK Price Variants (UKPVs) as part of the initial print run. These are not reprints - they were printed in the same plant, on the same press and at the same time as the US versions. The UKPVs have a cover price in UK currency (9d when distribution began). From 1960-1964, the cover month was removed so as not to appear out of date once they'd reached UK retailers, and a line was added to the internal indicia noting that the comic was printed specifically for the UK distributor.
For books cover-dated between December 1967 and March 1969, cents-priced Marvel Comics were imported for sale in the UK. T&P ink-stamped the covers with a UK price, as was their standard practice with the DC comics they also distributed. In April 1969, some time after Marvel had moved all of their printing to the World Color plant in Sparta, Illinois, UKPVs resumed.
T&P did not import the whole range of Marvel titles, but tended to bring in the majority of the super-hero books. There are also multiple short-term hiatuses in both production of UKPVs and in the stamping regime. I'm working on trying to put together a semi-definitive list of books that weren't distributed in the UK, but as you might imagine this is likely to be a long and arduous process. Steve Cranch has put together an actually definitive list of printed UK price variant Marvel comics here.
Current anecdotal evidence suggests that Marvel comics went on sale in the UK in one batch per month rather than weekly, as in the US.
While T&P were sole distributors for the vast amount of titles in this period, L. Miller & Son exclusively distributed five titles from May 1960 to August 1961 - Amazing Adventures, Gunsmoke Western, Rawhide Kid, Two Gun Kid and Wyatt Earp. In addition, it appears that Gold Star Publications (GSP) imported some cents-priced titles (most likely significantly after they would have regularly appeared on shelves) during a gap in T&P's regular distribution in 1966, possibly caused by T&P entering receivership in July of that year.
June 1971 - January 1981 - World Distributors
Based in Manchester, World Distributors took over the distribution contract for Marvel's US comics with books cover dated June 1971. Thanks to the vagaries of Marvel's on-sale dates vs cover dates in this era, there is some crossover with the last batch sent to T&P - cents copies which were price-stamped by T&P on the front cover - and the first batch sent to World, which had a printed UK cover price. World did not use ink stamps on their comics - all comics imported by World had a printed UK cover price.
Unlike Thorpe and Porter, who primarily serviced independent newsagents, World were a national distributor who were able to distribute books into larger chains like WH Smith.
As in the Thorpe and Porter era, there are a number of hiatuses in production of UKPVs during World's tenure as distributor. The number of titles offered by World in any given month tended to hover around the mid-teens - far from the full range of comics that Marvel offered. One reason for this is that Marvel UK heavily restricted the titles that World were allowed to import to avoid sales dilution of their black and white UK reprint comics, beginning with The Mighty World of Marvel in 1972. Consequently, many titles and individual issues in this era were Not Distributed (ND) to UK newsagents. This practice largely ended when Comag took over the distribution contract in 1981.
Beginning with books cover dated August 1974, the 'Marvel Comics Group' banner at the top of the cover was replaced on the UKPVs with a 'Marvel All-Colour Comics' banner, to further distinguish between the homegrown black and white Marvel UK titles and the imported American originals. The last comic to feature a 'Marvel All-Colour Comics' banner was Captain America #250, cover dated October 1980.
The final batch of books distributed by World were cover dated January 1981.
A significant quantity of the black and white magazines that Marvel published in the 1970's - including Tales of the Zombie, Deadly Hands of Kung-Fu, Doc Savage and Rampaging Hulk - were distributed in the UK by GSP. Copies distributed to newsagents carried a UK price sticker on the cover. At the request of Marvel UK, who launched their own version of the black and white Savage Sword of Conan magazine in the same year, this arrangement ended circa 1977. At that point the production of black and white magazines in the US was winding down anyway.
April 1981 - December 1995 - Comag
Comag, co-owned by publishing giants Hearst and Condé Nast, were a significantly larger business than T&P or World. As such, the range of Marvel US comics they began importing into the UK market was almost double what World had previously offered. The first batch of comics distributed by Comag were cover dated April 1981 - no Marvel comics cover dated February or March 1981 were imported to UK newsagents. For comics cover-dated April 1981 to December 1981, UKPVs were produced. For comics cover dated January 1982 to October 1990, Comag imported the standard Direct Sales editions of Marvel comics, with dual US/UK prices printed in the corner box.
By the mid-1980's, Marvel comics were on sale in UK newsagents about one month before their cover date (e.g. January cover-dated books were on sale in December).
Beginning with comics cover dated November 1990, Comag switched from importing Direct Sales editions with printed UK prices to mostly Newsstand editions with a price sticker applied. Initially these were unobtrusive price gun stickers. From late 1991, some UK distributed books carried no price sticker at all, and some carried a large, often poorly applied, barcode sticker on the cover.
As Moore Harness did with the DC books in this era, Comag also distributed to speciality shops in the UK, though they hit the shelves some months later than the air-freight imports the shops were buying via direct market distributors. This was especially useful for retailers when a recent title became a hot commodity in the direct market. Though the direct market distributor may have long sold out of a given book, Comag's books - transported from the US by sea rather than by air - would still be in plentiful supply,
The final batch of Comag-distributed books were cover dated December 1995.
January 1996 - January 1998 - IPC Marketforce
IPC Marketforce continued to use large ugly barcode stickers, very similar to the ones Comag had been using, primarily on Newsstand copies of Marvel books. The latest book that's been found with one of these stickers is X-Men #71, cover-dated January 1998, though as of this writing no other books from that month have been unearthed. UK newsagent distribution of other Marvel titles may have ended with December 1997 cover-dated books.
Though Panini had started publishing various Spider-Man and X-Men reprint titles in the UK by the time Marketforce took over distribution of the US comics, this doesn't seem to have affected which titles Marketforce was importing, which were very Spider-Man and X-Men heavy.
The IPC Marketforce era of UK Marvel distribution is almost completely unresearched and, I'd guess, universally unloved. Were it not for a change in the classified listings in industry paper Comics International in January 1996, I'm not sure anyone would even know that Comag stopped distributing Marvel comics into newsagents and Marketforce took over.
Coupled with declining sales in the non-specialist mass-market, Marvel's return to exclusive distribution through Diamond Distributors in the US in April 1997 may have spelled the end of newsagent distribution in the UK for US Marvel comics. When DC went exclusive with Diamond in 1995, Portman's UK newsagent distribution operation was heavily curtailed. Marketforce may have been unwilling to agree to a reduced slate and to end distribution to specialist shops, and simply dropped the line entirely instead.
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Though not technically a newsagent, mass-market bookstore chain Borders did carry a selection of US Marvel comics distributed by Diamond UK. As far as I can remember the comics that Borders stocked were standard Direct editions and carried no additional stickers or stamps. Borders opened their first UK store in London's Oxford Street in August 1998 and collapsed into administration in 2009, with all of their remaining stores closing by the end of that year.