Defra has released this advice on caring for farm animals and pets during the cold weather, including this note for horse and pet owners:
Where pets (such as rabbits and guinea pigs) are normally kept outside in hutches during mild winters, eowners should consider moving them into garages / sheds to provide additional thermal insulation. Where cages cannot be moved additional protection or insulation should be provided wherever possible.
It is also important to ensure a supply of drinking water. Ice should be cleared from drinking water containers and the spouts should be defrosted regularly.
As with farmed livestock, horses and ponies usually kept outside during the winter should have access to shelter at all times and a regular provision of feed and water ensured. Where such provision is inadequate, owners should consider moving the animals and/or permanently stabling in the interim period Water supplies for all horses and ponies should be checked regularly and alternatives sources supplied if mains failure occurs.
In an emergency you should contact the RSPCA for help and advice. Read their advice on animals in the cold spell here.