You live in the UK where it rains a lot. Horses in dry cold cope a lot easier than horses that get rained on as the rain flattens the horses coat and they can get soaked through, and this cold.
So to answer your question,it depends on the following:
Do you work your horse sufficiently hard and often to need to clip him in the winter? If so, then you should rug.
Does he come in at night, or live out 24/7?
When he is in the field, does he have good shelter from the rain? If yes, then you shouldn't have to rug.
I'm an advocate of not rugging when unnecessary - but if your horse is in a field with NO shelter from rain for 24 hours a day then in that scenario even I would be rugging. Or preferably, moving to a field/paddock/yard with shelter.