Mountainbiking.
Yes the ridgeway is under ankle deep mud, but the peak will kill a set of brake pads before you stop for a sausage roll at fairholmes, swinley turns to grit an to off piste gets destroyed, Scotland is covered in snow, the Lake district is just wet, as is wales. Everywhere can claim that they’re the worst.
Singlespeed, small rotor and sintered pads (runs hot, minimises pad wear), decent lights and get out and ride.
I actually enjoy the winter riding as much as the summer, it’s so completely different it my as well be a different sport, but the fitness transfers perfectly. Just needs a bike and kit that suits it.
I only road ride in winter when we get a big blocking high pressure and there’s no chance of sailing (another sport where the winter series is a different challenge).
Other than that, I always plan to get a head start on the DIY when the weather’s crap so I can get out more in the summer. Ditto the garden, get all the pruning done and the borders dug over and weed membrane over the top until the frosts are finished so that doesn’t need touching. That way when the weather is actually nice I can get out!