I've been to Alton several times for their "Winter" openings in January and February. We're just not prepared for the cold. I managed an hour I think before having to give up on several occasions simply because your ears, feet, hands and face are painfully cold - never mind the kids. I have lots of photos of snow at Alton
Of course, if there was somewhere to shelter inside and have a hot drink to warm up before heading out again, it would have made a massive difference. It's just not made available though.
Add in the fact our road and public transport systems just cannot cope with the slightest adverse weather and the parks just aren't ready for this at all.
The parks that seem to stay open (or have special winter opening days) are those that are near the larger conurbations. Adventure Island, Gullivers, Twin Lakes and Drayton (limited rides) for example. Though it's worth noting that they're also quite "kid friendly". I suspect that families are always on the look out for some kind of entertainment like this even in the winter. It's okay, as long as they don't have to drive too far out of the way down nasty little roads.
Of course, somewhere like Thorpe should also thrive, but it seems to me that we're just not geared up to a rolling maintenance procedure. The big parks are stuck in a "closed all winter for complete check ups" mentality, rather than scheduling it correctly, or using the long summer evenings when the park closes bloody early!