Hello Stephanie, welcome to the forums, hope you enjoy your stay.....
It may surprise you, but your sennies probably don't need any extra heat to handle the winter, even if it were to snow....there are breeders in the Netherlands that break the ice every morning on their bird's water dishes.....
Our feathered friends are able to fluff their feather layers separately, to control their body temperature and the way their legs and feet are veined, it would take sub-zero temps, with the bird not moving for an extended period before the cold would affect their feet.....I don't recall the UK getting that cold in recent history, so I think that as long as they have somewhere to get in out of a chilling wind, they would be fine.....
In the event you are still of the opinion they need some sort of heat, a controllable heating pad would work, as long as they don't try chewing the pad and/or the electric cord.....