I hope none of your friends or relatives are coming from or through London for Christmas.
Airlines are not canceling flights only delaying them so people keep showing up for their flights. The airport operator, BAA, has just asked airlines to officially cancel flights and are asking passengers to not show up at the airport since they are completely full of people already. The latest estimate is that no more than 50% of flights will operate between now and the end of Wednesday. Gatwick and most UK airports are in not much better shape. Several regional airports are still closed.
It’s amazing that a major airport like this can’t get more than one runway open. BAA are saying even tomorrow Heathrow will still have only one operating runway.
TGIK went to Heathrow Saturday, spent all day there going nowhere. Went back Sunday, was told to return after Christmas.
Why is BAA so bad at handling snow? Cancellations at non-BAA airports are minimal.
The good news is the Government is going to form a commission to study the problem. 
It happened last year too. And I think the year before that. To be fair this is the most snow they have seen for quite a while.
Knowing how the CAA works they probably demand 0 snow or ice on the runways and taxiways before they will open them. Looking at the film on the news the runway and taxiways that are open look pretty well clear of anything.
The good news 
is the rail network is buggered (to use a British term) too so you might not even be able to get to the airport anyway.
This is what I’m talking about:
EGLL 201550Z 10009KT 9999 BKN042 01/M02 Q1001
And they still won’t get one runway open through tomorrow.