From HSE website:
HSE previously defined thermal comfort in the workplace, as: 'An acceptable zone of thermal comfort for most people in the UK lies roughly between 13°C (56°F) and 30°C (86°F), with acceptable temperatures for more strenuous work activities concentrated towards the bottom end of the range, and more sedentary activities towards the higher end.'
I did a full day with temperatures peaking at 32 degrees.
Personally I found it uncomfortable to work in, but being self-employed I have little choice.
Mind you I also don't fanny about having cool boxes, factor 50 and flip-flops....