Brilliant - thank you so much for the suggestions. This is exactly what I was looking for: pretty places like Corbridge, where the good light trails are, theatre.
I didn't know any of those places!
Which of the light trails would you suggest if we were only going to do one? Chatsworth on the way (staying over one night on the way up), Stockeld Park, Alnwick Garden or Gibside?
We visited York a couple of years ago, so I'd discounted it - but it looks really great for Christmas so maybe we should go back. More into general Christmas atmospheric than shopping.
We do like the outdoors when the weather is good - but yes, I'm afraid we're a bit soft for blustery winter beaches! If we were lucky enough to get snow that would be different 
Theatre and carols would be great. Will look at Hexham. Anywhere else that's known for kids theatre, dance or music? We're seeing a panto down here, so probably not panto.
Beamish museum looks great. That's right up our street.
Science museums are always a hit, especially hand-on and experiments. Magna outside Sheffield looks cool, but might be shut for winter. We've been to a few planetariums, but haven't yet done actual star gazing: is that possible anywhere in that area, given the lower light pollution?
Thanks again everyone! If you need tips for Surrey, just ask 