I'd never recommend Nicholas of Camden. Have not heard very good manners and have only been in there once and although I wasn't interested in anything at the time so don't quite remember it, my husband does remember their customer service not being very good*
Jacobs have some overpriced used gear,
Aperture have lots of nikon stuff but I can't afford anything there either. Aperture do have a very nice cafe though so I do visit that if I'm near the British Museum
*I think for legal, ass-covering, reasons I'm being coy.