I managed to fit this in to a long day trip with breakfast at Stonehenge, lunch in Bath and getting to Bleanavon for the last tour of the day! Bleanavon makes a lot out of its UNESCO status there were banners all through the town and it was nice to see as many places in Britain don't make much of a big deal of it!
But there is a fair bit to see here the main site "Big Pit" is interesting as you get to go into the mines with the people that used to work there as your guide. There seems to be a really community feeling to the valleys of South Wales, and it was not a surprise to find that the whole thing is free!!! Especially in an area of Britain where Socialist morals were once so strong.
It is easy to drive to Blaenavon as it is easily marked from the M4 which runs from London, if you do not have a car apparently the No 30 & 23 buses run every 30 minutes from Newport (easy to get to from Cardiff). If you are in the area it is worth exploring Cardiff which is rapidly becoming a major cultural centre in Britain and also if you carry on about 5 miles north of Blaenavon you are in the Brecon Beacons which are one of the nicest natural areas in the British Isles, ideal for hiking and camping