We recently had a couple of threads on this, available from the search function:
http://www.flyertalk.com/forum/briti...ng-out-t4.html
http://www.flyertalk.com/forum/briti...xperience.html
To be honest, if you are travelling First class on Christmas day, there will be no traffic whatsoever at that time of night, so I would book a car with Addison Lee and aim to arrive at T4 as close to the flight closing time as possible. You will find it a creepy, spooky, haunted ghost town. Drink copious amounts of champagne upon arrival at Gate 1.
Ironic for a premium travel experience to the longest long-haul destination, but there we are.