ACAS have some guidance on 'hot weather' working. The guidance confirms that:
'In the UK there is no maximum temperature that a workplace is allowed to be, rather advice from the Health & Safety Executive (HSE) states 'during working hours, the temperature in all workplaces inside buildings shall be reasonable'. What is reasonable depends on the type of work being done (manual, office, etc) and the type of workplace (kitchen, air conditioned office, etc).
The HSE offers further guidance on workplace temperatures including details on carrying out an optional thermal comfort risk assessment if staff are unhappy with the temperature - Health and Safety Executive (HSE) - Temperature.'
The ACAS guidance also covers issues such as getting to work, keeping cool at work, fasting during hot weather, vulnerable workers and dress code during hot weather.
Internet link: ACAS website