I am not currently living in the UK so hopefully there will be someone along shortly with even better answers but I get a lot of new books from the ww.Bookdepsitory.co.uk and used ones from the amazon marketplace.
Carboot sales and secondhand bookshops are also great. Doesn't Education Otherwise also sell things?
There is plenty on the internet to download for free as well. I will provide links if you like.
I wish that there was a one-stop-shop that is inexpensive and if it exists, I would love to find it...