For outdoor pigs, I'd recommend stuffing the bedroom of the hutch with hay, as much as possible. Check it every night before bed, to make sure it's dry. If it's damp, replace it or top it up. The pigs will tunnel into it and hopefully be really cosy.
For indoor pigs, a big cardboard box filled with hay would be good.
If you only have one pig, then remember they won't have anyone to cuddle up to to keep warm.
Snugglesafe heat pads are microwavable and meant to stay warm almost all night. I can't say if they're good or not but I'm going to get them this year and try. I'm only going to use them at night during the winter. Because someone said not to use them during the day, or the pigs won't get used to the cold, and won't be able to handle it as well.
Wooden hidey houses are better than plastic I think, because the plastic won't be as insulating, and might get condensation inside and be damp. Some people say plastic ones hold in heat better, but I'm not sure, I'd use wooden ones, or a cardboard box, and stuff the hidey house with hay too, but with enough room for the pigs to get in.
If you have a few pigs, and if they'll share a hidey house, I think it'd be a good idea to have a house big enough for all of them to get in so they can cuddle together. But have a spare one too in case one pig gets kicked out. And if they're in a hutch, be careful that one pig isn't being kicked out of the bedroom and sleeping in the open part. Mine have been sleeping out there in the summer sometimes, but in the winter they'd be freezing.
Also animals need more food in the winter, so make sure they have lots of high energy food to build up a good layer of fat!
Corn would probably be good if they're not too fat already
and mine will be getting lots of pellets, maybe unlimited cos I know I'll get worried about the cold. (They're still mostly under a year.)
I'm going to have my pigs in the shed for the winter. It won't have any heating and isn't insulated or anything, but hopefully they'll be ok and it'll keep the wind and rain out. I live in the south of Ireland and it's quite mild here, but remember that different parts of the UK can get quite extreme temperatures, so if you live somewhere it gets really cold then it'd definitely be best to bring the piggies inside. I couldn't believe it got to temperatures like -14 where my grandmother lived in reading . . .