One of my favorite cities in the world is Bath England. The city is located at the edge of the Salisbury Plain and has preserved much of its ancient heritage. Bath is the only thermally active area in the UK. The existing Roman Bath structure was constructed approximately 1000 years ago. Some of the Roman plumbing and walls date back to the year 1100. The water still flows and is available for drinking and tastes absolutely terrible, but what the heck, when in Rome...
The city of Bath is definitely worth spending an entire day exploring. In addition to the Baths themselves, the city itself is beautiful and the ancient canal system and locks that run through the city are still in use today and worth seeing (there is a walking path along the canal).
For a bit of historical culinary flare, try eating at the world-famous Sally Lunn's. Sally Lunn was a French woman who came to Bath 300 years ago and sold a unique bread that was never quite duplicated. The restaurant has a variety of dishes that incorporate the original recipe.
The city of Bath is about a 90 minute train ride from London (the train station is within a 5-10 minute walk of the Baths and downtown area). If you travel on a Sunday the train tickets are generally half-price. You can also drive or take a bus to get there (about 2 hours). Other nearby attractions around the Salisbury Plain include Stonehenge and the Salisbury Cathedral, which houses the original copy of the Magna Carta.