Holland and Barrett is the one in most towns, but I don't think it does gelatine powder.

This is from amazon uk:

http://www.amazon.co.uk/Chef-Willia...wdered+gelatine

Still really expensive. I think you can arrange to have it delivered somewhere to collect near where you're staying if that helps - if you're willing to pay the price.

Lots of galleries in London, Tate and The National Gallery are great.