A very good and dedicated online service, specialized in UK hotels, is Active Hotels.
By clicking on this link you'll be able to find a hotel in London, at London airports, or elsewhere in Britain with an interactive map:

Active Hotels - London & UK hotels interactive map

And here's the link to Active Hotels Special Offers:
London & British hotels current special offers

A second specialized service is Venere - Internet Accommodation Reservation.
To visit it, click here:
Venere - Hotels Reservation Services.

Venere directory of London hotels, apartments & other accommodation

Another service is Accommodation Line:
Accomline list of London hotels