For the general population, 18C to 20C is the temperature range which should be aimed at in offices, in schools and in homes. (Anything much above 20C tends reduce people's ability to function at their best).
However (again for the general population) when people are actually in bed (rather than, say, using their bedroom as a study), there's no real need for any heating in bedrooms unless, perhaps, the temperature is close to freezing outside. (I've certainly never heated a bedroom).
So 18C would seem to be a rather high temperature in a bedroom (where people are actually in bed) and not particularly unacceptable at other times. Elderly people might require slightly higher temperatures (when they're not in bed), but not substantially so.
Public Health England
used to recommend that people heat their living rooms to 21C and their bedrooms to 18C but they've now revised their recommendation to suggest that 18C is perfectly acceptable throughout for most people:
https://publichealthmatters.blog.gov.uk/2014/10/21/preventing-avoidable-deaths-this-winter/
However they do note, in that document, that some people might require rather higher temperatures.
There is no statutory minimum temperature in any workplace but the Health and Safety Executive works to a guideline figure of 16C:
http://www.hse.gov.uk/temperature/faq.htm#minimum-maximum-temperature