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.