Piggies are most comfortable in temperatures around 18-20 degrees but provided they are kept draught and damp free and as warm as possible inside a hutch then it is possible to keep them outside year round, it just takes a lot of work and preparation.
You will need a very substantial hutch - thinly made plywood hutches arent really substantial enough to cope with a UK winter (weather damage will occur) and don’t keep draughts out well enough. Hutch size is also important.
Mine have a hutch inside a shed but have constant access to the shed floor to play. My shed unfortunately cannot be heated but the shed itself obviously protects them from the elements. My efforts therefore go into keeping the hutch warm as it’s obviously easier to warm that smaller space. To that end, in winter they get locked into the hutch at night.
If you have the possibility of moving them into a shed or unused garage then it would be good.
Inside the hutch, they have the hutch filled entirely with hay. They have four snugglesafe heat pads between the two piggies (once heated they last 8-10 hours so are popped back in the microwave morning and night to keep them warm at all times). Snugglesafes are essential for outdoor piggies.
Damp hay being removed and replaced twice daily in winter.
On the hutch I put a thermal hutch cover, then two blankets, then another hutch cover (this one is waterproof but obviously I don’t need it for it’s water proofing qualities given they are in the shed, but it acts as a valuable extra layer). I then throw an old duvet over the whole thing.
There is a thermometer on the hutch so I can monitor what is actually going on under the covers. I also have a plan to bring them into the house if these measures fail. Mostly these measures work well but this winter they did fail and it was too cold for them to stay out.
I actually have more trouble keeping them cool in summer than i do keeping them warm in winter
Cold Weather Care For Guinea Pigs