Jolli Joe is a bit of an enigma. They are all metal, (possibly cast), painted Blue, and they have a label on the top that says "Jolli Joe the UK Uke". They are pre WW2 but still fairly common. The only firm date I have found is one in a 1935 catalogue from the Birmingham distributor F. R. Martin. 

To go into more detail on the construction - They appear to be made of 'Zamac', a family of alloys with a base of zinc with added aluminium, magnesium and copper in varying proportions. Aluminium content is constant at 4%. Zamac was developed by The New Jersey Zinc Company in c.1929. Zamac can be painted or electroplated. These ukes usually appear to be painted with a 'wrinkle' paint finish, that is often seen on old electrical component boxes or other utility metalwork. This wrinkle effect is achieved by applying a thickish layer of an alkyd-resin based paint, which remains relatively soft for some time during the drying process. A layer of quick-drying cellulose lacquer is then applied on top while the paint is still soft and as the lacquer dries and shrinks the wrinkling effect occurs. It would appear they were spray-painted, judging by the 'overspray' traces on the inside surfaces of the body. Flaking paint is also a common feature of these instruments and could be due to insufficient pre-treatments or to micro-corrosion of the alloy surface causing localised surface instability. Other causes are possible and long term paint stability was probably not considered a priority for these economy instruments. They are normally a denim-blue colour, but sometimes they appear green. The 'green' appearance is due to yellowing of the cellulose lacquer over time. The yellowed lacquer over the blue paint gives the illusion of green. It is possible to remove the discoloured lacquer to reveal the original paint colour, but this is a process best left to experts. A similar process is employed in paintings conservation, where a discoloured surface consisting of dirt and old varnish is removed and renewed with a clear coating to enable appreciation of the original image. (thanks to Chris Egerton for all of this information). 

There is no maker's markings other than the headstock plaque and they don't appear to be from any of the major UK Banjo factories. However, whilst a lot of them come with their own faux crocodile case, I have seen a few with a more standard John Grey case, and though it doesn't directly say it is one, F.R.Martin only distributed John Grey Banjos? If they were part of Rose Morris, (who owned John Grey at this time), I don't think they came from any of the usual London factories?

There was a Banjo Mandolin version of this made as well, called the Mandy Jo and it was usually brown.