Model shop Leeds, Sussex Models, Gliders UK and several others. Google will get you any others. Stock of common speed build kits is poor in all of Europe right now. Remember UK foamboard is all 40% heavier than FT stuff, which is sold by the sheet at most of the above places.
There’s a UK subforum of FT here
https://forum.flitetest.com/index.php?forums/uk.157/
Lots of other info there.