Michael Page (or their sister company Page Personnel might be better for someone starting out in Finance) and Hays are both large recruitment agencies with good contacts within the profession. I have experience of both (all 3 if you treat PP as a seperate company) of these companies and have been very impressed with each.
There will also be other agencies operating in your area (as you are in London I expect there to be several hundred). Also try the CIMAjobs section of this website. Even if there aren't any jobs that suit you on there, it will give you more agency names to send your CV to.
Good luck!
