Hmm, depends. An official official White Christmas is one flake of snow on the Met Office roof in London during the 24 hour period as Rekstout said, but there are regional variations, esp. for Scotland and the North of England. Certainly in my area we tend to take Leeds more as gospel than London. It was a White Christmas in our area on my 18th birthday and an official one (London) on my 21st!
Thanx for the answers guys, don't think i'll rush off to the bookies just yet though lol like rekstout said it's a pretty safe bet with London being generally warmer !