It will cover only certain things, expensive crowns and fillings which are not covered by the NHS will not be covered or will be simply be over the cap of your yearly insurance coverage.
If you need a simple crown that is subsidized if not completely by the NHS you'll be able to offload it to the insurance pretty easily, if you want the top of the line ceramic crowns that cost 1500-2000 GBP no insurance will actually cover that.
