Hi, Whilst December is not a month i would choose to head up to the loop it could possibly be a rewarding one IF you are are lucky with the weather! Maybe contact one of the regulars via flickr or fighter control.
Flying still happens in December though how much can again be a matter of luck i'm afraid. If you plan a couple of days on the hills and have a good hip flask then that will help increase the chances of some action i'm sure.
Either way, spending some time in Wales with a loved one is a stunning month as are the preceding 11.
Good luck!