International Supply Teachers charges about £35 for online registration. As it sounds, this agency deals with short-term vacancies, such as for maternity leave or to cover for an unexpected vacancy. They generally want quick turnaround, so you must be able to mobilize within one to two weeks: pack your suitcase now! Emails are sent out whenever a suitable position is available, at least every two months.
This can be a good option if you are feeling in "halfway house" mode: not sure where to go next. You will always have a limited contract, so you can't get stuck in a bad situation for too long, and it gives you the chance to experience a variety of places, cultures, systems, school types and sizes etc. which should stand you in very good stead in the future.
Apart from the individual short-term contracts with each school, there is no obligation to be a supply teacher for any minimum period, so you can take advantage of any opportunities which arise to turn a temporary position into a permanent one.