Research the practice of hypnotism and consider how you would like to apply your abilities as a hypnotist. There are many UK websites devoted to the topic of hypnotism that can provide the background information needed prior to official training. The Hypnotherapy Association and the British Society of Clinical Hypnosis are both good sources for reading more about hypnotherapy.
Find a training course, either online or traditional, that suits you and sign up. If you wish to become a hypnotherapist, choose a training course that specializes in the therapy side of hypnotism. If you wish to become a stage hypnotist, choose a more general hypnotism course and consider classes in theater as well. HypnosisEducation.com and Hypnotherapists.org.uk provide a lists of various hypnosis training courses that are available in the UK. One possible training course available is provided by the London College of Clinical Hypnosis which offers training courses throughout the UK as well as advanced degrees in hypnotherapy.
Teach yourself if you think that learning on your own suits you the best. Purchase training books, audiobooks, or DVDs that teach hypnosis techniques. One resource you may use is the BBC contributor Derren Brown's "Hypnosis Tricks of the Mind" audiobook.