I'm curious about that, too.  I don't think I've heard that particular number, but I've heard several times that a lot of people freeze to death in the winter in the UK because they can't pay their bills.  Why would that be?  It's not a big problem in the US, and we offer a lot less of a social safety net than the UK.
On average, it is colder in the UK than in the US.  That's not necessarily a statistic, just my opinion from having lived in both places.  It is also more damp, and the housing stock, on average, is significantly older.
So is there any consensus that this is a realistic number?
The anecdotal evidence Nick Rouse gives below (old housing combined with lots of archaic and hard-to-maintain coal heat) seems plausible.  

I also agree that the 50,000 upper-bound excess mortality rate in the winter is foolish to ascribe entirely to hypothermia.  As just one example, in the US 30,000+ people (mostly elderly) die of the flu every year, so scaling to the UK that would be about 6,000 from flu alone, the great majority of them in the winter.