As far as I know this is similar to my partner's deal and the most expensive thing is a crown and only the fabrication of that is not included, the rest of the treatment for it is in the plan.
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.
I also pay that and both AXA and BUPA cover upto 75% and upto like 300 and something GBP a year unless you are doing procedures which are also covered by the NHS.
Edit: Pretty sure I pay £30 odd a month and I've had root canal treatment, new crowns, etc.