The Roman Baths are situated in the centre of Bath next to the Abbey. It’s not possible to swim there. But, there is also a spa in Bath, where you can enjoy natural thermal waters.
Apparently, the Roman Baths are one of the most visited heritage attractions in the United Kingdom.
You can easily combine your trip with Stonehenge or London.
When I lived in Bath the baths were open for swimming during the Bath festival and well worth the experience.