I thought the link I posted was more recent than that one!
Here's a cut & paste from what I have contributed previously on this topic:
There is a legal minimum temperature (13 Celsius if my memory serves me well) but no maximum for a "normal working environment."
However, there are certain clauses in the Health and Safety at Work Act which you can exploit to get relief from the hot environment. One of these is that you are entitled to remove yourself from a work area if you believe that your health and safety are at immediate risk. So, if you are feeling light-headed or otherwise affected by the heat, you can legitimately claim that you are in immediate danger (from fainting/trips/falls etc.) and remove yourself to a cooler area for a spell. Note that the law does not require you to prove you are in immediate danger; you merely have to believe that you are. So, use it to your advantage.
If your work area is not normally so hot (that is, it is a consequence of the hot weather), then your employer is duty bound under the law to take all reasonable measures to moderate the conditions. For example, an office worker's normal working environment does not require them to swelter under 80-odd degrees heat. If it did, then the office would be heated to such temperatures during the winter months. It is therefore unreasonable for an employer to expect office workers to work in such heat without taking all reasonable steps etc.
Of course, if your employer employs less than 6 workers, the H & S at W Act will not apply.