Hi,
I suppose it depends what you mean? According to
Wikipedia It is frequently said that "birding" is observing wild birds for pleasure, "birdwatching" is amateur study of birds, and "twitching" is seeing rare birds to accumulate a long list of species seen worldwide or in some given area.
So I suppose most of the UK population have at some point have been Birding and possibly even birdwatching ......... even if it's absent mindedly staring at 2 squabbling blue tits on a bird table!
Now twitching, well that’s a different matter. According to the book "
The Big Twitch" there are about 4000 active Twitchers in the UK with an estimated further 3000 who Twitch occasionally.