While it is a bit far for me for a day trip, if I really wanted only one day out of London, and could not do an overnight, it would be Bath.>

I heartily agree and I've done it as a day trip a few times - just 90 minutes or so by train each way - well that I understand for many maybe too far but for me the train ride is also interesting and you see some of the British countryside (as well as a lot of industrial detritus however along the rail line).