LilyLolo.co.uk (mineral makeup)
Zuneta.com (for harder to find luxury cosmetics like Edward Bess)
The ones marked with * above all have free shipping.
Most major cosmetics brands have online stores on their websites, usually if you subscribe to their e-newsletters they will send you offers/codes from time to time (e.g. for free shipping). Good ones to try: MAC, Bobbi Brown, NARS.
Most major department stores also sell cosmetics online. Debenhams do free delivery to Beauty Club members and deliver instore free, House of Fraser do free delivery to store.
I've only ever used tried E.L.F for online makeup shopping.... I can't say the quality was that great. Depends what you are looking for I suppose. I like their nail varnish though, it went on nice and easily
Definatly elf, the quality can be a bit hit & miss but some of the stuff is really good for the price. Especially the face primer, mascara primer & HD face powder. Big fan of the studio line of brushes & the mineral lipsticks too.
I've ordered MAC, Nars and estee lauder from cheapsmells and I've been pretty happy with what I ordered. I know people have had problems with the quality of products from buyapowa and possibly being fakes.