lljkk I'd dispute your assertion about the 'British definition of vegan' and think it slips into the 'veganism as ideology' and even towards the 'vegansim as cult with top-down direction' trap. The key question is whose definition of vegan are you talking about?
Vegans are people who don't eat animal products. It really is that simple. (I was one, in the UK, for many years and didn't feel the need to explain it beyond that. Though I could explain my own particular, personal reasons for making that choice and, how far I chose to extend it into other areas of life).
I do agree, as above, that there is an association with ethical choice. The 'accidental vegan' might not describe themselves as such - mainly, I'd have thought, to avoid the sort of tedious wind-up whataboutery that a certain kind of time-wasting omnivore likes to indulge in, to make themselves feel good, or just because they're an immature twerp. Though they would when indicating food-choice for an airline meal or a wedding.
It may be that there's a new wave of very ideological vegans. It may be that The Vegan Soc has changed its wording. But, neither of those groups owns the word vegan.
The Vegan Soc is not The Pope. It is a membership organisation that represents the interests of its members. That's all. The majority of vegans are not members of that or any 'vegan group'. They're just people who are vegan.