Good plan to think ahead, but we've noticed in Darkest Essex that the nights are fair drawing in
I'll list all the things I've tried with varying degrees of success 
Deep hay -as in the haybox full to the brim. This is what my boars liked best to keep warm.It gets peed on and needs changed loads.
I bought some lovely lush hay for them , but it must have had alot of seeds, GP1 had a weepy eye on Sunday. DD and I hoiked out a grass seed. He's fine and it hasn't put him off his hay but now I've taken it out of the haybox and put it in a trug.
In the haybox they've got...
Fleece: nice and snuggly but the demons pee on it, it gets stinky and you need to brush the hay off before you wash it. I put a folded fleece in the haybox and a cardboard box with a fleece inside so they've got their wn space,
So far, so good. GP1 eye fine.
Snugglepads- solid silicone discs, microwave. They have a fleece cover. They stay warm about 8 hours but they're hard
I used hot water bottles but only in a waterproof pillowcase protector. I use new ones every year to reduce the risk of rubber decay. (You can wrap in newspaper to insulate)
Pet Hotties- soft gel microwaveable pads. Don't fit in my microwave without alot of elaborate tucking in of edges and don't keep the heat.
Tubular heater on wall - about 12" long. I could honestly get more heat if I stood with a lit match 
Last year I bought them a small oil filled radiator. OK it needs electricity. And something high enough to stand on (and keep out of the way of piggies who could chew the lead) But it was the best thing.
Only thing was I had to put it on 30 minutes before they went out for the day. And it switches itself onto 'standby' when it reached temperature. Which
me at first "My pigs, they'll freeze"
Straw is a good insulator but not recommended for guiineas (I use at the back of my Haybox but not while Dill is a minipig)
And if you get a baby piglet, give it some alfalfa hay (but not for the adult)
Good luck with your new arrival. It makes a huge difference to a bereaved pig. We saw to GP1 "How do you cope" when GP3 popcorns and runs about. If he had eyebrows, they'd be like this \ /