My understanding of the 20m figure is that it represented 80% of the TV viewing audience at that time. Which is pretty impressive to me, given the wide choice of things to watch on TV, 4 out of 5 chose the football.
As interesting to me, is the indication here of the very large number of people
not watching television. Figuring very roughly (my maths, utterly awful) -- UK's population checked via Google (no offence meant) as a little under 70 million: this would seem to indicate as, at Euro 2020's very peak, 25 million watching
something on TV: whence 45 million not watching TV. Discounting those "out of the equation" -- as mentioned by an earlier poster: babies / small children; people in nursing homes -- or otherwise critically ill; or people at work, in jobs demanding a high level of concentration; shift-workers asleep (and not choosing to forego sleep in order to watch the game); or doing essential non-paid-employment stuff, more urgent than footie-watching; or homeless and marginalised, without access to a television; and discounting all who for one reason or another, were voluntarily TV-less in that time-window -- this would seem to leave a mighty lot of people who could have been watching something on telly at the time when when the biggest game was on, had they wanted to -- but seemingly they didn't want to.
We're often accused as a nation, of overwhelmingly suffering from "square eyes": the above would seem to throw some doubt on that view of things.
How many people watch it because they don’t want to be the odd one out at work, or be accused of being unpatriotic, or they don’t want to be cold-shouldered in the pub (*)? It has become like religion in the days of the puritans: anyone who doesn’t join in is to be ostracised.
* - Some years ago a friend stopped using his local pub after over a dozen years after the new landlord started making jibes about the fact that my friend hadn’t even realised
that England were playing on one occasion.
"Conform, or be despised and rejected" -- one of humankind's widespread nasty traits.