Thermae Bath Spa is the only place in Britain where you can bathe in natural thermal water, and the rooftop pool, open-air, warm at all times, surrounded by Bath's honey-coloured skyline, is one of the most genuinely special experiences in England. You book a two-hour session (though they don't enforce it strictly), and the building also contains indoor pools, steam rooms, and treatment facilities.
Book ahead, this fills up fast, especially at weekends. The rooftop is best at dusk or in cold weather when the steam is visible. Go early or late in the day rather than lunchtime for the most atmospheric version of the experience.