I've found that the Internet is a great place to get started when looking for a job. I've compiled a list of sites I found helpful while looking.
www.jobcentreplus.gov.uk - Government run agency. They're very helpful with finding a job.
www.rec.uk.com - Most agencies have a little REC symbol meaning they belong to this confederation. This site is fantastic, helps you find agencies and even states your legal rights when using an agency. Very important to know!
www.secsinthecity.co.uk - Well, I just put this in because the name is funny. It's an admin based agency in London.
fish4.co.uk/jobs/index.html - I really liked this site. It was helpful in finding me agencies in my area.
www.monster.co.uk - If you can afford it, I totally recommend signing up for the Premium account. It comes with a lot of information in finding the right job for you as well as tips and aid in interviewing, cv's and such. Plus, I found more agencies contacted me after I had got the Premium account.
And the last:
www.yell.co.uk - The Yellow Pages. Just put in employment agencies. It'll come up all the agencies in your area. Most of them have websites included with the information. I printed out all the names, phoned and signed up with them all. Best way to get a job: put yourself out there.