Best of luck with your plan, Dan
I have loved pigs since I first worked with them on an intensive unit in the 80s. Finally achieved my goal of having free range pigs of my own three years ago - and can confirm that the wet cold weather in these parts (I'm far NW of England, north of Hadrian's Wall and about 20 miles south of the Scottish Border) is not nice for pigs. You need good winter accommodation for them, especially any youngstock. To my surprise, my Saddleback x weaners wintered indoors very very happily last winter - they had a large pen with a big round bale of hay to burrow into (not planned, it just happened that way and they loved it so much we left it there for them - they had just about used it all up by the end of the winter
) The pen was at the entrance to the cattle shed, so there were people coming and going a lot; they enjoyed the interaction and always seemed completely content. This year I don't have homebred youngstock, and my two bought-in Large Blacks do not like being indoors - get bored and fiesty - but get cold if it's too wet and cold outside, even with a good shelter and lots of dry straw. So I have had to juggle giving them outside time but bringing them in before they get too cold. Their outside pen has been a swamp for a good week now, so yesterday I gave them the run of an old hen run, currently unused, for a few hours. Happy sleepy pigs last night when they came back in - but there's not enough grass in there to keep them going for very long. If the ground doesn't dry up a bit I shall have to resort to taking them for daily walks!
I've had to open up another part of the field for the adult pigs - it's no fun plodging up to your belly in mud, Meg-pig tells me, especially when there are wee stones buried in the mud that go up between your cleats and hurt your feet when you walk. Bless 'em, they set off to 'work' in the far end of the field each morning after breakfast, returning tired and happy at tea-time.
My pigs are just a hobby and for food for us and family and friends - no way could I do real free range properly here without a lot of thought and a huge amount of ground to allow for rotation.
Tammies would be very hardy, probably even more so than my OSB Meg-pig, but I would certainly be looking towards the drier parts of the country for a free range setup.
Best of luck!