Ever thought of a weeks surfing? even if your a complete novice, just messing around on a foam board in the white water with a bona fide instructor will do wonders for your fitness and conditioning for the elements. It can even be done in fairly wintry weather. But caution as you will need to suit up adequately warm so instruction here and equipment provision is the key. As for getting your stamina up and fitness if you choose the colder months) that aside its a whole lot of fun, it is really hard to beat.
Then indoor snowdomes help get the proper muscle memory into place (and the right muscles working) a week or so before you actually arrive at your chosen ski destination.
Note there is a ski slope at Llundudno in North Wales, not that far from Manchester) which is an outdoor offering and pretty cool trendy too. ski or snowboard on offer there. it's a dry slope last time I looked (a year or so ago).
Gym is also worth keeping in the mix as this is most likely the nearest fitness training facility you will have nearest to you.
Ever thought of skateboarding. No not the spinning kickflip tricks sort but just slalom boarding? it gives you a workout, gets the hips and legs moving in a similar way to skiing)
I reckon you get the picture by now.
I would consider one of the above alternatives along with the ski slopes training thrown in to keep the muscle memory working and conditioned. Hope some of this helps. (dont forget nutrition, to keep it all intact)