Be sure your dress is businesslike if you are going for office jobs, as London isn't as casual as other places. Be prepared to have your skills such as typing, spelling, grammar, Maths, computer knowledge, etc. Some agencies are more thorough than others, so registration can sometimes take a few hours. 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. You will need a contact phone number, as when work becomes available you will be contacted immediately to fill the position.