I have to go to Reading once per month with my job and I always buy a roll or two of film from the little independent camera shop near the train station. It isn't cheap - but I like the novelty of buying film in a shop, they have a good range of Ilford and Fuji, possibly only 35mm.

For mail order, I find Mather's of Lancashire and ag photographic to be brilliant - fast, polite, well priced:

http://www.ag-photographic.co.uk/
http://www.mathersoflancashire.co.uk/html/film.html

If you you don't mind waiting a week or so for your package, 7DS are very competitively priced:

http://www.7dayshop.com/catalog/default.php?cPath=777

The Fuji pro lab are quick to post, but you should check their postage charges as they can mount up. Until recently, they beat 7DS on price for some 10 packs.

http://www.fujilab.co.uk/catalog/