There is an NHS dentist in Camden High St that I went to once ... but I wouldn't recommend him ... his equpiment was about 40 years out of date. Sorry that isn't very helpful is it. Your choice of NHS doctor is limited to those within your "area" which is usually only a couple of miles radius of where you live, so if you are in W. Hampstead, you may not be able to go to the same doctor as someone living down the road in Swiss Cottage, for example.
Aimiloo, NHS dentists are not usually free (except for children and pregnant women), but cheaper than a private dentist.