What Roman bath complex in the UK is one of the best-preserved ancient baths?

The Roman bath complex in the UK that is one of the best-preserved ancient baths is the Roman Baths in Bath.

The complex developed around a natural hot spring at Aquae Sulis, the Roman settlement now known as Bath. A temple was built beside the spring around 60–70 AD, and the bathing facilities expanded over the following centuries. Roman engineers stabilized the watery ground with oak piles and lined the spring chamber with lead.

The site included a sacred spring, temple, bathhouse, and museum. Its bathing rooms followed the familiar Roman sequence: the hot caldarium, warm tepidarium, and cold frigidarium. Hypocaust heating circulated hot air beneath some floors, while the spring supplied naturally warm water.

A common mix-up is treating the modern buildings as entirely Roman. Much of what visitors see above ground dates from later reconstruction, especially the 18th and 19th centuries, but substantial Roman remains survive below street level. The baths fell out of use after Roman rule ended in Britain during the early fifth century.