i hate to go on about it, cause i say it in every answer, but call the PALS service at your local Primary care trust, who will be able to tell you where to register, or put your postcode into nhs choices and click on dentists then expand it and they will tell you if they are taking on new pts. Also, PALS will be able to give you the dentists on the emergency roster (should be covered each day) for pts in your situation (ie need emergency treatment but dont have a dentist) they will be able to give you a temporary filling till you can get in with the new dentist.
i would also urge everyone to register with a dentist (nhs) even if they dont need it now. A check up is about 15 quid under nhs charges and at least youll be registered if you need it to avoid emz's situation