Many agencies prefer you to ring first for an appointment, but others don't mind if you walk in off the street for an interview. Be sure your dress is businesslike if you are going for office jobs, as London isn't as casual as other places. Also bring your passport, your tax form and any other documents that mention your work status, as they may need it.
Be prepared to have your skills such as typing, spelling, grammar, maths, computer knowledge, etc tested. Some agencies are more thorough than others, so registration can sometimes take a few hours.
You will need a contact phone number, as when work becomes available you will be contacted immediately to fill the position. If you do not have a phone number, you may miss out on a job. Ring the agency at least once a day in the morning to advise them of your availability and ask about any potential jobs they may have for you.

Payment and Schedule
When you are being interviewed and registered, ask about the rate of pay you can expect to receive. Rates are usually determined by what skills you will be using on the assignment. Some agencies offer lower rates for new arrivals. Be sure to ask if the rate includes holiday or sick pay. If you are going away on a trip or holiday, be sure to let the agency know for how long, so they can work around your schedule and find you an appropriate assignment. If you accept a long-term assignment knowing you will be leaving shortly to go on holidays, you will be letting your agency down. It could affect future assignments.
Payment from agencies is weekly or bi-weekly, on receipt of your signed timesheet, which you usually bring or mail to the agency. They will credit your bank account, or you can collect your cheque at the agency office. They usually prefer paying directly into your account, but many agencies allow you to cash cheques at a particular bank without a bank account. But be sure to ask your particular agency about this. Many jobs on offer at temporary agencies become long term assignments. Many employers look for future employees this way, so you can sometimes get lucky and get a permanent job.
There are literally hundreds of agencies in London, so be sure to find the ones that are right for your experience and interests.

An agency we recommend for job search
Aansoek.com