The Roman Baths get very busy, but one way to avoid some of the crowds is to visit during the summer late opening.
The Museum and Baths themselves are open until 10.00pm every evening in July and August (last entry is at 9.00pm). Bath tends to get lots of day trippers so visiting in the evening is much more peaceful experience.
It doesn’t get dark until about 9.30pm at the beginning of July, so you might want to consider that when timing your visit. The Baths are lit by torches and it is actually very dark indeed after the sun goes down. Please watch your footing. No tripods are allowed on the UNESCO World Heritage flagstones so its a good test of camera ISO performance. It’s also quite a challenge to balance light levels of the Baths and the Abbey and sky above. I had some fun with the visitors using selfie sticks.
This year there were several “Romans” wandering around only to happy to chat about life in Roman Bath and pose for photographs. When I visited with my sons a centurion offered to demonstrate his sword on me!