I posted this on another thread; assuming you're looking for PA/Exec. Asst. type work:
The first agency I signed up with, who gave me my first job, was ACME (and no, Wylie Coyote is not the Office Manager... my US friends have already commented!) - it was for only a day, but, it paid well, it was in a great area, and this was my first week here, and I appreciated that the woman trusted me right off the bat. (I think they have a few offices, I went to the one near Oxford Circus)
http://www.acme-appointments.co.uk/ Another one that was a bit flaky, but a good friend of mine has had better luck with them, and I did get one very good, long term, well paying gig, was Roberts Knight Selection (down the street from Bank station).
www.robertsknight.com The two I LOVE:
Hodge Recruitment - a bit slow starting, but ended up with another longterm temp assignment in the West End that was FAB; and the job I'm in, which was a temp to perm, and I LOVE IT. The women at Hodge are lovely, patient, and positive.
www.hodge.co.ukHudson - I registered and interviewed with them the day before I got my present job; so I cannot say I have firsthand experience at their placement; however, I was more than impressed by their facilities (HUGE building and offices), the organisation - one person tests, one 'coordinates', one 'fills the positions', as well as a very extensive interview, really wanting to know the specifics about me and what type of environment I would suit and vice versa - and, they were the only agency that made a point of emphasizing, CALL US EVERY DAY.
I was highly, highly impressed with them, and they would be the first I would call should I need to change jobs.
http://uk.hudson.comI also have heard that Office Angels is one of the most popular agencies as well.
Good luck!