My favourite Christmas TV movie has got to be The Flint Street Nativity, it's an ITV one that was on a few years ago but, as far as I'm aware hasn't been on since, though you can buy it on DVD.
It's fantastic, absolutely hilarious. It's full of British actors (Frank Skinner, Neil Morrissey, Jane Horrocks, John Thompson, Stephen Tompkinson, Ralf Litte, Julia Sawalha and Dervla Kirwan to name but a few!) playing children in a school nativity. I think it's a must for anyone who works with kids as they just get the playground politics down perfectly (e.g. the spoilt little girl who wants to be Mary) whilst still being funny and very christmassy. Hopefully they'll put it on TV again soon!