There are three main ways:

First, use the British Medical Journal (BMJ) Careers section. Every week there are hundreds of open posts available that you could apply for. Note that the deadlines are relatively short, i.e. about two weeks.

Second, use a locum agency that will organise a job for you. You'll find ads of locum agencies every week in the BMJ or on the internet. However, some locum agencies are trying to fill less attractive non-training posts with foreigners because these doctors don't know what to expect. Be careful what you sign up for.

Third, apply directly at the hospital you would like to work at. Ring up the medical staffing department or the consultant. Sometimes posts can't be filled because someone fails his / her exam or is off sick. You might be lucky and fill the gap before the job is advertised.