The websites that I described below sell American sweets to British people at a good price because they are British stores that sell American sweets. This is good because there's almost no other way to pass on American-exclusive edible goods to the British, who might enjoy consuming American goods they never heard of before.
American Fizz (americanfizz.co.uk)
This website has a retail store in Bournemouth. It's open between 10am and 7pm on Sundays to Fridays and 9am to 7pm on Saturdays. When you order £50 or more of goods on this website, you'll get free delivery. They also sell drinks and other types of food, not just American sweets/candy.
American Soda (americansoda.co.uk)
Just like American Fizz, when you order £50 or more of goods on this website, you'll get free delivery. They, unlike American Fizz, also sell Japanese candy and non-food items.
The American Candy Store (theamericancandystore.co.uk)
With plenty of types of groceries, drinks and sweets/candy, it looks like a good place to buy American sweets/candy.
Candy Hero (candyhero.com)
You can get American sweets/candy from this website, as well as sweets/candy that come from the UK, Finland, Sweden, Australia, New Zealand and Japan. It's likely to find some sweets/candy that suit you.
A Taste of the States (atasteofthestates.com)
Tasting American edibles is what you should do before you die, and this website can give you plenty of that, including with certain hampers that you can buy on the website.
eBay (ebay.co.uk/bhp/american-sweets)
Millions and possibly billions of other items are sold on eBay, but this is just a specific page of American sweets.