I had a root canal done, which was unsuccessful. This was in Australia, so the procedure might be different. The root canal itself was fine, not at all painful, in fact I fell asleep in the chair! The first check up 6 months later was fine. It was the check up 18 months later that they found a hairline crack all the way down the tooth, and there was nothing that could be done to save the tooth and I had to have it removed. Sometimes it just happens.
It was the very rear molar, and the one just before it was going to have a filling put in. The specialist immediately contacted my dentist and told him to put a crown on that tooth and not to fiddle around with a filling, as it would have more pressure put on it because of the removed tooth.
I had a normal crown put in, and it always felt odd. The dentist had got the size so badly wrong that he had to file all the white capping away down to the metal inside anyway and it really looked awful. Then several years later I changed dentists to one who specialised in white fillings and I had all of my fillings changed to white, and the crown changed as well. They had to send it away to switzerland for the mould to be done. It worked out much cheaper having it done in Australia than in England (Aussie dollars!!), which was why I had it all done at the same time on a trip back home (I'm originally from Australia and was visiting my family).
If you want white fillings, and a white crown, you have to find a dentist who specialises in them. Apparently the procedure between amalgam and white is really different, and those who do amalgams tend not to be as good with white fillings as those who specialise in them.
Hope this helps. Sorry I can't tell you what the NHS would do.