Hi there.
My piggies live in a shed outside & I'm also in Lancashire
have you insulated your shed already? If not i would suggest to do this first (walls, door & base). You can get a roll of it online & spray glue. This will keep the heat in. I have 1 heat source which is a cobbled together fan heater on an extension lead & I give it a good blast in the evening. Its inportant to maintain ventilation too so a window which you can open a bit (securely) to let air circulate is important to get rid of any condensation which may build up (it occurs over a few days if there is not enough circulation, just from the piggies breathing.
You'll probably find a better heat solution than me as I'm waiting to get an outside socket installed but its ok for now.
Snugglesafes are good for the night plus at least a hidey each stuffed with hay (i cover my hideys with a blanket, or t towel or fleece just to keep it snug.
Personally i would spend the next couple of weeks getting it snug (and seeing off any February snow) first and then introduce them. I'm preparing myself to bring them inside if the temps drop dramatically so the last thing you want is to get them out only to bring them back in again
.
They will be absolutely fine though once they are in there, it really is just the condensation issue you need to be on top of especially if its insulated to the max like mine is
....if youve got a run you could also think about getting a tube connector to make things even easier (if u do that let me know as i found a much cheaper option than the runaround tubes
).