"Last year saw the largest increase in UK deaths in a single year since 1940, according to provisional ONS figures"So that's more than The Blitz during WW2, then.
I assume the media is using this blog post: https://blog.ons.gov.uk/2021/01/12/counting-deaths-involving-coronavirus-a-year-in-review/
Which clearly states that it's the largest year on year increase since 1940. The later was years would therefore have been working off the higher 1940 baseline.
However the ONS blog post states that they don't hold details of overseas military deaths and this page states 1940's peak was caused by rising infant mortality: (https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/articles/trendsinbirthsanddeathsoverthelastcentury/2015-07-15).
Meanwhile google suggests the blitz killed about 43,000 civilians in the 12 months from mid 1940 so relative to Covid-19 it was a fairly minor increase in the numbers.
Today's blog post has some other figures I find interesting too. 2020 was only the second year on record with over 600,000 deaths. The previous one being 1918, although the population was only half current levels back then.
Also of note is taking into account age and population size 2020 was only the worst year for deaths in just over a decade (since 2008).
That last figure appears to suggest that as a population we have been enjoying a boom of longevity lately.