Does anyone know a website which is specialized in selling DSLR cameras?
I live in UK, so UK based retailers would be preferred.
Could you tell me what type of DSLR you are looking for? What price range?
You can check out canon ebay outlet (a few times I have seen some crazy discounts on there). You can also try cameraworld.
I'm looking for Canon EOS 5D Mark III and Sigma 70-200mm f/2.8 EX DG OS HSM lens. If possible the price range for the camera (body only) should be ~£2000 and for the lens ~£800.
Currys/PC World are your best bets these days. As Jessops is gone, there's not much left on the high street. Unless you want a second hand, in which case give CEX a try. If you do go down the route of getting it through Currys/PC World (the same company: Dixons PLC) they can always order it in.
Good luck! The Mk. III is a great camera!
Jessops are still around, I work at a Currys/PC World and we have one across the road. I would say to check to see if there is a Jessops near you as well then maybe go there or to a Currys to see if you can test one out. At Currys the 5D Mk III is a bit more expensive than what you are looking for but if you were looking to get a lens and some other stuff with it then usually we give a little bit of discount. At least we do in my store.