We're lucky enough to have a nice dedicated RPG store in Birmingham but there's a selection of core rule books and various supplements for RPGs in the Waterstones book store too.
Waterstones doesn't have a vast selection but last time I perused their shelves they had games like Traveller, D&D, Call of Cthulhu, Trail of Cthulhu, Battlestar Galactica, Dark Heresy, Star Wars, Serenity and others.
I know that not every town's Waterstone bookstore carries the same stuff but you could give them a try.