Unfortunately, you need to buy your film online now. It's normally difficult to get hold of in shops, so most people who use a lot, buy online, so local shops stock even less... etc.

A few advertisers in AP offer discount film, you normally get a fair discount when you buy 10 rolls of each at a time. mailshots and discountfilmsdirect are both AP advertisers IIRC

http://discountfilmsdirect.co.uk/shop/acatalog/Fuji_Superia_100_36.html

Ł16 for 10 rolls here