Firstly the bad news is that there is no maximum working temperature in law.
However, employers are expected to provide a "reasonable" temperature at work, and therefore need to assess factors such as temperature, humidity, air velocity, how physical the work is etc, and have sensible means of coping with high temperatures.
Sensible measures can include air conditioning or fans, ventilation, cold drinks, plenty of breaks, and cooler materials where uniforms/overalls are worn.
If you have air conditioning units installed in your workplace, the system should be the right one for the size of your workplace. Use a competent installer to ensure that it is installed in accordance with the manufacturer's instructions, so that it works safely and does not introduce additional hazards to a workplace i.e cooling some staff while heating others - hot air from air conditioning units should usually be vented outside, not into another work area!
Air conditioning units count as equipment provided for work, so they should also be properly maintained and inspected by engineers to ensure that they continue to work as intended.
If you don't have air conditioning, drink plenty of cold drinks, and take frequent breaks etc.
And if you're working outside, don't forget the sun screen.
The relevant health and safety law is :
Temperatures - the Workplace (Health, Safety and Welfare) Regulations 1992;
Risk assessment - the Management of Health and Safety at Work Regulations 1999;
Machinery - the Provision and Use of Work Equipment Regulations 1998.
These links may also help :
http://www.hse.gov.uk/work-equipment-machinery/puwer.htm and
http://www.hse.gov.uk/temperature/thermal/index.htm
The weather forecast indicates the hot spell isn't due to last, so make the most of it while you can!
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