I have visited Bath on several occasions now and it is a really lovely city to visit, quintessentially English, but famous for its Roman roots. The whole city is made of local sandstone and it looks very impressive, especially if you manage to visit on a sunny day when the warm creamy stone seems to glow and liven up the whole centre. The interior of the Abbey is well worth a look as it has some impressive fan vaulting.
Perhaps one of the most impressive features and the most famous are the Roman baths, and I would strongly recommend paying the high entrance fee to see them, they really add to the whole experience and there is a lot more to see than just the central pool, the remains of the roman temple, which lie underneath the main square are particularly impressive.
Much of Bath's lay out and current architectural features are from the Neo-Classical Georgian period, which had a huge impact on the design of a huge number towns in Britain as well as further abroad (Edinburgh and Monticello/ University of Virginia are other WH sites where you can see this). Two of the most impressive examples of this style are the Circus and the Crescent which are a little further up through the town, providing a nice walk.
Bath is very easy to get to, it is on the main train-line from London to Wales easy to get to from the south as well, if you have a car it is just off of the M4 which runs from London, (just off of this road is Avebury which is part of the Stonehenge World Heritage site as well near Swindon). Bath is a nice day trip from London, and is impressive due to its uniformity.