The Workplace (Health, Safety and Welfare) Regulations 1992 says that your employer must maintain a reasonable temperature where you work, but it does not specify a maximum temperature.
There is a minimum temperature of 16°C, or 13°C if your work involves considerable physical activity. However, your employer is also expected to prevent your workplace being uncomfortably hot. There should also be enough thermometers around the workplace so that you can check the temperature.
But whatever thermometers read, if most people are complaining of the heat, common sense says that it is too hot and something must be done immediately. Remember that how we respond to heat can also depend on the weight and age of a person.
You should also remember that air temperature is only a rough guide because humidity, wind speed, radiant heat sources, clothing, etc. all have an effect, which an ordinary thermometer will not take into account. It is possible to get a more accurate assessment using specialist equipment such as a wet bulb global thermometer or electronic equivalent, which measures humidity. The comfort range for humidity is between 40% and 70%.
The TUC has called for the introduction of an upper limit on workplace temperature so that employers would be forced to act when the temperature inside reaches 24°C. It would mean that staff could be sent home and their employers prosecuted if temperatures at work hit 30°C (or 27°C for those engaged in physically demanding work). The TUC has set out the case for a legally enforceable maximum temperature.