I've just been on the Dental Protection website - Full DPL Membership for Dental Nurses (working more than 20 hours per week) is £74
Discounted membership - where a dental nurse is employed in a general or specialist practice by a DPL member or employed in a hospital (NHS or University), community, dental school or armed forces is £37
With regard to the Hypnotherapy qualification - I studied with Uncommon Knowledge at Brighton University in 2006 and qualified DipHypPsych (UK). I basically did the course because I wanted to help people with dental phobia. I had no qualifications apart from 'O' grades (levels) gained at school and the National Certificate in Dental Nursing. Before starting the course there was a "taster weekend" which enabled potential students to assess their suitability to do the course. I enjoyed the course - people of all age groups and walks of life including one dentist were my fellow students.
Since qualifying I, sadly, haven't used hypnotherapy at work but I do see people privately.
Uncommon Knowledge no longer does the training course which I did but one of their tutors does a very similar course under the name "Within Sight".
Human Givens College - does a Diploma in Human Givens - this is based on attending day, weekend courses, a fortnight "school" and a series of practical sessions.
Unfortunately these courses tend to be rather costly - I think that you would need to have private work at the end in order to re-coup the amount spent.
I know that the British Society for Medical & Dental Hypnosis does courses but I think that they only accept doctors or dentists on to their courses.
In addition to that it is reckoned that their are about 250 hypnotherapy schools throughout the country.
Sorry to have provided you with such a long answer.
Kind regards
Janet