The law doesn't specify a specific temperature.
Regulation 7 of the Workplace Health, Safety and Welfare Regulations 1992 simply requires that a 'reasonable' temperature must be maintained in indoor workplaces during working hours. The 'Approved Code of Practice' which accompanies the Regulations (but does not form part of them) states that the minimum temperature where the activity is mainly sedentary should be
16C. (If the work involves significant physical effort a lower figure of 13C is given).
http://www.hse.gov.uk/pubns/indg244.pdf
Chris