Whilst there is a legal minimum temperature for indoor workers (it's 16 degrees celsius - or 13 for strenuous work), there isn't actually a minimum for people working outdoors.
However, this doesn't mean that people who work outside are totally unprotected from the extremes of the UK climate. Under Health and Safety regulations, your employer owes you a 'duty of care', which means that they are responsible for ensuring you don't have to work in unsafe or unhealthy conditions. Continuing to work in extreme cold could well constitute a breach of this duty of care.
If you're concerned about the temperature you're working at, talk to your union safety representative if you have one, or contact the Health and Safety Executive for more information.
