It definitely depends - on a lot of things. The sort of population of the county, the amount of money the PCT is given to use on dental contracts, how many dentists in the area decide they want to take part, etc.
In Buckinghamshire there are pockets that have loads of available dentists and pockets that have none. But if someone rings us we can pretty much always find them something - they may have to go to the other end of the county but they'll definitely be seen.
The other thing to bear in mind is that the restrictions on being on a dentist's list are not the same as a GP - you can see a dentist near your office, or near your relatives, or wherever - you aren't restricted to dentists in your immediate area....