Winter UK
December
As ever at this point of the forecast confidence does tend to slip away, so please bear this in mind...
At this point in time the first month of winter is expected to continue in a similar vein to November in terms of the overall setup with the northern blocking continuing. Subtle variation on that theme means that currently the expectation is that the month as a whole will be slightly cooler than average with rainfall close to the normal for the time of year.
http://www.netweather.tv/index.cgi?action=longrange;sess=
You'll need to keep a check on the above site because it will be updated as time passes and more weather info becomes available.
Bear in mind that what the link is saying right now is that temperatures in December will be 'below the seasonal norm' which might indicate snow.
If snow is likely at LHR on 12th December and you have time, you might be able to switch to another airline and arrive/depart at/from another airport here in UK which is not snowbound.
LHR snow
http://www.youtube.com/watch?v=1CZXFOH68og&feature=related
Take my advice. UK does not know how to deal with snow. Our normal weather is for short sharp cold snaps from late December into late February or early March. Snow is not common, but when it does snow, we do not know how to deal with it. Bring on the Canadians.