As you're in the UK I'm pretty sure that this is only happening overnight. Last thing in the evening, make sure the water's completely topped up. In the morning, break the ice and re-fill. It will have only iced over for about 4 hours overnight and your horse won't have suffered for that short space of time.
I'm assuming the water is in a bucket? If you have automatic water feeders look out for problems of frozen pipes!
Remember to break the ice on top of the water troughs in the field too.
Never in my time horse-owning in the UK did I or anyone I know have to resort to water heaters in our horses water. The cold is so temporary that it doesn't become a real problem. I doubt the type of water heaters that other people are thinking of are available there.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.