Yeah, that's pretty normal - I'm due for an NHS dental check-up this month but I wasn't very organised and should have booked it months ago, so I'll probably have to wait another couple of months or more for an appointment (best thing to do is book your next appointment for 6 months later when you attend the first one).
For now, I would check if your BUPA covers dental, because if so, then you could book as a private patient and it won't cost so much.
Otherwise, if you're not willing to pay out of pocket (and £200 is a lot of money), then you might just have to wait the 6 months for an NHS appointment.