OH who does counselling work says hypnotherapists don't have to be registered, but the better ones tend to be accredited eg like in 1ozzy's link.
If you are in distress, you could also ask for or seek out a number of other therapies as well, such as CBT (widely offered through the NHS) and NLP (neuro-linguistic therapy). Find a list of accredited CBT practitioners here:
http://www.babcp.com/Default.aspx
NLP is only loosely regulated, but the 'INLPTA' accreditiation gives some quality control. Occasionally in NLP a practitioner may suggest trance work, but not as a first off.