Whilst there is a legal minimum temperature for indoor workers (it's 16°C, or 13°C 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. This 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.
Extra risk assessments should be carried out during extreme weather, to make sure workers' health and safety are not put at risk. These should focus not just on the outside air temperature, but also on the wind chill factor. Strong winds can make it feel bitterly cold in the open air in the UK.
Your employer must ensure appropriate protective equipment is issued and that where necessary, warm and dry mobile facilities, and regular hot drinks are available.
Rest breaks should be more frequent, and long enough to allow you to warm up properly. When it comes to personal protective equipment (PPE) remember that feet and toes are particularly susceptible to cold injury. Appropriate protection of the hands is also important not only to prevent injury, but also to maintain dexterity and prevent accidents.
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 (HSE) for more information. The HSE website has more information on workplace temperature issues.