Are there any UK supermarket-brands that make kosher cakes or biscuits?
Some branches of Sainsbury’s and Tesco have a small kosher section, which will do cakes and biscuits. Depends on your location. You could always phone your local store and ask. Otherwise, look for standard packs of biscuits, cakes etc that have a small logo that consist of a K in a circle. It means they’re certified kosher. Lots of mainstream products have it.