SAA is a fair shout. Think you need to be a member to get the cheaper prices, which are roughly comparable to Cass Art.
Sometimes Jacksons have good deals. If spending over 30-40 quids a look at artdiscount as potential savings are made with free-postage, dependent on spend. Whst do you need, student or artist quality? A starter set or particular pigments?? I just shop around for the best savings/prices on whst I need at the time I’m looking to buy paints, they all seem much the same as far as online retailers go.