There are a couple of very good sites for buying diamonds. The best (certainly considered by the diamond trade) is www.bluenile.com An American site, but you can buy from the UK. A friend bought a diamond from there a couple of years ago, and got an excellent deal. There is even a system of being able to 'build your own ring', choosing size of stone, style of shank, setting etc. The majority of their stones are certified too, so you know what you're getting.
Certainly BlueNile have a range of D IF stones which are GIA certified (Gemological Institute of America - one of the forefront gem testing laboratories in the world whose certificates are very much trusted in the trade).
Alternatively, if you are looking for a solely British site, have a look at www.holtsdiamonds.com They have a store in Hatton Garden, so you can have a look at what they have in stock online, reserve something, and then come up and have a look. My engagement ring came from Holts, and I cannot praise them, or their service, highly enough.