Yeah, that tallies with my experience - WhatsApp is the 'lingua franca' of group messaging in the UK (which I'm left out of as I don't like Meta!).Yeah. When you look at the stats we actually have a high percentage of iphone users. Always at least 50%. Latest stats say 52% but iMessage is just not a thing. Everyone is on WhatsApp. The only person I use iMessage with is my mum. All the other iphone users I communicate with it’s via WhatsApp.
So it's funny when I read accounts such as in the source article from the US where 'no one wants to be the green bubble in a iMessage group messaging thread'...
...I think that most iPhone users in the UK don't even know that iMessage group chat is a thing (as per above, they just use WhatsApp for that).
And as you say, similar to you and your mum, the blue bubble is just something that appears when you message a friend/relative who happens to have an iPhone too - it's not something that people actively look to do.