Last minute question for our trip - what's the best way to get from heathrow to London (Kensington Gardens) on Christmas day?
Check out the Heathrow Airport website at http://www.heathrowairport.com/. The only public transport on Christmas Day will be a replacement bus service running from the airport to Paddington station. You would then need to get a cab from Paddington to Kensington Gardens, but there may be few cabs running. As a general rule, it is best to avoid arriving in Britain on Christmas Day unless there is someone able and willing to collect you.
I agree with Bob. I'd look for a cab at the airport and be prepared to pay. And pay.
Thanks for the help. We ended up reserving a trip through Green Tomato cabs which, while not as cheap as the Tube, at least wasn't charging double on Christmas Day.