There's loads of camera shops down Tottenham Court Road - go to Tottenham Court Road tube station and walk up the road from there - there's more camera shops there than anywhere else in London - all the shops are really close to each other as well so you can compare prices and ask for the best deal - the guys in those shops can be a bit pushy because the competition is fierce but you'll get a decent deal.

Also - near that tube station, there is a Jessops - come out the tube station and head down New Oxford street.

Jessops will generally have the best price but you can go into the other shops along Tottenham Court Road and say "I can get this from Jessops for £xx - what can you do it for?".

Also try the London Camera Exchange:

http://www.lcegroup.co.uk/

They have some really decent 2nd hand stuff there which is guarunteed - I bought a lens from there recently and saved myself £50.