"Receding front gums: what is covered on the NHS?"

I am under what I thought was a very good dentist, however I am on Income support as well as incapacity benefit and a single mum to two small children. I recently went to the dentist for my 6month check up I stated that some of my front teeth lower and upper gums are receding now showing my roots. And the front crown is now showing the root steel that holds this, I have been told I cannot claim NHS costs for the treatment and that I have to pay for each tooth a cost of £400 for each to be crowned?

I thought I would be exempt from these costs? When I look up what is covered through NHS it does not go into much detail like, when does it effect a patient to smile knowing she showing her roots and cant afford to pay for it.

Where do I stand with this I cannot find any information online?