Allcraft, in NY allcrafttools at yahoo dot comsells two thicknesses
of foil. The thinner can only be used after all soldering is done.
The thicker can survive a bit of soldering. Allcraft ships
internationally. Rio Grande also carries gold foil. Theirs is a bit
thinner than the thinnest that Allcraft sells. When I want thicker
foil, so that I can do a lot of annealing and soldering, I buy the
thinnest 22 or 24K sheet I can get, and then roll it out until the
rollers won’t roll it any thinner. (If you want it thinner yet, you
roll it between sheets of oxidized copper.) I hope this helps, if you
are unable to find a UK supplier.