Some will kick you off the book if you don't go for a couple of years and many people don't think to go yearly unless they have a problem.
There is also a major issue of getting on the books if you move to a new area, of if your old dentist retires/goes fully private.
They don't 'kick you off', in Scotland and Northern Ireland, the government kick you off, if you haven't signed a registration form with examination every 2 years, in England, the government doesn't even actively register patient's at all, so in theory if its been 3 months they don't have to see you, but most keep it around 2 years.
The situation is dire due to massive underfunding, and its worse in the regional areas, so many many dentists and practices in order to provide a proper standard of care to their patient's will go private, seeing a fewer number, and treating them better, either on plan or for a better level of remuneration.
They won't recruit the dentists they need as the dental number simply doesn't exist, and if you think its bad now, give it around 4-5 years, and you'll find a massive amount of the number will have retired in that time.
I think it is time to wind up NHS dental services altogether.
Have employed dentists, to deal with under 16s and in pain relief clinics, who simply take teeth out, and everything else you pay for, nothing to do with the government, as they've demonstrated they have no clue how to administer anything or future plan or achieve anything in the way of results.
For disclosure purposes, I am dentist, I've worked 25 years, I've in that time never one done a year of less than 90% NHS activity, and frankly now... I'm done. NI dentistry is bloody awful, the fees are appalling compared to even England, and many of the treatments I am doing for patients of 20 years history with me I am doing for zero profit, it can't continue, people need to be paid, people need to earn money. Hilariously do to some way they jig the figures, and have split the old NHS pension into different schemes, I'm going to get a massive tax bill for breech of AA this year. It makes absolutely no sense. A very sorry situation.