This should do the trick... your can't beat the The Offices, Shops and Railway Premises Act of 1963
"Employers must ensure that during working hours, the temperature inside buildings (including offices) is "reasonable". Under the Workplace (Health, Safety and Welfare) Regulations 1992, there is only a minimum temperature stipulated, which is 16 degrees Celsius after the first hour of working, unless the work involves strenuous physical exertion.
Whilst there is no maximum temperature ceiling legislated for, the British Safety Council cite research which concludes that when people experience temperatures in excess of 24 degrees Celsius the propensity for accidents increases and work productivity diminishes. The World Health Organisation (WHO) recommends that the maximum air temperature should be 25 degrees Celsius. Safety Reps’ can use these figures and evidence to negotiate agreements that improve upon the minimum requirements of the law.
The test of what constitutes a "reasonable" temperature is inherently subjective; it would be fair to say that if the majority of the workforce consider they are too hot or too cold for most of the working day, then the temperature is unreasonable.
There exists no legal right to vacate the workplace as a result of extremes of heat and cold, unless there is "serious, imminent and unavoidable danger" (Management of Health and Safety at Work Regulations, 1992). But there is no reason why workplace reps should not negotiate a joint agreement on temperature, which sets out what will happen when the minimum and maximum levels are not maintained.
Some suggested measures to achieve satisfactory working temperatures include:
Insulating pipes
Providing air cooling plant
Shading windows and siting desks away from radiators and other hot spots
You should be aware that under The Offices, Shops and Railway Premises Act, 1963 in office premises where "members of the public are invited to resort" or any room which constitutes "railway premises", then the minimum temperature of 16 degrees Celsius need not apply unless it is "reasonably practicable".