I posted this to M4 the other day to see if helps him get work, and have just seen Rac3r's thread too, so figured it might be of some use to the people who live in the UK. It is an unedited long list of names of agencies. I didn't compile the list alone so can't give you much more info on who they are or what they do, but you can Google their names to see if they have a website (which i would have thought they must):
Badenoch & Clark
Birchwood Knight
Bsm
Community Safety Skills
Covent Garden Bureau
Deekay Technical Recruitment
Destination Education
Dma Group
Dragonfly
E J Church
Excel Recruitment Ltd
F1 Recruitment
Federation Of Small Businesses
Forties People
Fraud Jobs
Fresh Minds
Fusion People
Harris Hill
Hays
Hays Sales & Marketing
Hunters Recruitment
Intelligent Resources
Kellys
Macaulay Search
Manpower
Media Contacts
Meet The Real Me Ltd
Michael Page
Morgan Hunt
Morgan Law
Morgan Spencer
Nfp Resourcing
Norma Skemp
Office Angels
Online Resourcing
Page Personnel
Pfj
Policeskills.Co.Uk
Prc
Profiles Creative
Prospect Us Recruitment
Prospero Teaching
Quantum Recruitment
Real Recruitment Limited
Recruitmentrevolution.Com
Reed
Regan & Dean
Regent Selection Ltd
S Recruitment
Think Search And Selection
Timberline
I'm not too sure if they are all London based or not.