What species? What kind of enclosure? Sun or shade? Dry or damp? Windy or still? There is no simple answer. All of these things are factors.
Hi Hayley
And welcome to another Brit. I live in Kent
Things are cooling down right now,, but the right answer depends very much on the size/age of your tort.
A big old tort like Joe would still be out day and night right now. It was over 20C here today and the ground temperature in full sun is very much warmer than the recorded air temperature in the shade.
Have you seen my thread about keeping a tort outside in the UK? It may help you with your questions
https://tortoiseforum.org/threads/outdoor-accommodation-in-a-colder-uk-climate.140866/