As well as the chain bookshops (eg Blackwells, Toppings, Waterstones, WH Smith) there are lots of independent bookshops. If you are interested in books get along to as many bookshops as you can – they often have very different personalities (and stocks of books!).
You can search for bookshops on The Booksellers Association Search website.
You can also support indies by buying from them direct online or through Bookshop.org. The biggest online bookseller is Amazon.
And of course you can often buy books in other retailers such as supermarkets, visitor centres, gift and other shops, and at events.
Book retailers are mainly supplied with books by publishers/distributors and wholesalers.