The figure of 50,000 looks like an upper bound of the "Excess Winter Mortality", defined as winter deaths (deaths occurring in December to March) minus the average of non-winter deaths (April to July of the current year and August to November of the previous year).  The UK figures are available from the Office of National Statistics, and vary between about 20,000 to 50,000 over the last 14 years, dependent on the severity of the winter.
If you're right, then Leggett is being, well, let's face it, baldly dishonest. Of course more people die in the winter. But it can't all be laid to hypothermia, for God's sake. It's the flu season, sick people get sicker, depressed people get depresseder (sorry), drinkers get drunker, roads get slipperier . . . It seems obvious that Leggett's Finnish zero is being compared with some quite different British 50, 000.