I will be in London for 10hours on a connecting flight. Any suggestions on what to do and it is advisable to leave the airport or can I get into town It will be Christmas Day
<<It will be Christmas Day>> wow - that's a key factor.
Please can you give more specific info - these are also key factors -
Which London airport do you arrive at, and what it the exact flight arrival time ?
Is it a true-through-connection on "one" through-ticket? Or have you bought separate flights in and out ?
If it's separate flight bookings, will you have hold luggage or just carry-on ?
It would have been harder to pick a worse day or a worse amount of time!