It seems to be the case now that supermarkets here no longer stock film, or if they do it certainly won't be black & white film. Branches of Boots are the most likely places in towns to stock film, now that camera shops are rapidly disappearing. Whether they have black & white I don't know. They certainly used to carry some Ilford & sometimes Kodak. There is a Boots in Bury St Edmunds at 11-13 Cornhill. Why not phone them from London and see what they have? Tel 01284 701516.