Jackson's Art are an online art supplier - although they do have a shop as well in London.
I always go to Jackson's first - because when i've had the time to shop around in the past they've always had the best choice and prices, for the things I tend to buy (linocut stuff, oil painting and watercolour supplies). Very satisfying to use up reward points every now and then as well! I ordered yesterday and today the materials have arrived, beautifully packaged in eco friendly packaging and card... absolutely faultless service. Also love that they often use DPD who are super helpful in letting you know when they're going to arrive! Great website too - loads more product info on their blog as well.
I am an art teacher and part time artist and shop with Jackson's pretty much every month.