Camera equipment in London is so expensive that you will probably make a profit simply by buying any camera here and selling it over there again as a used one.
In addition to the inherent high cost of the equipment, you also get to enjoy being slammed withh 17.5% VAT, which you can redeem at Customs (but have to pay a charge per receipt, so it's quite pointless unless the rebate is really huge).
If you are into Nikon and Leica stuff, you can try Aperture Photographic near the British Museum. It's also a cafe (the food isn't bad), so you get to drool at Leicas and Nikons whilst you eat. Their website is www.apertureuk.com
If you still must get a camera, try the reputable camera chains, like Jessops and Jacobs.