My random guess... British women say "boys" because they are boys... boys who just think that they are "men"... but they're really boys. Just a random guess.
Nah it's arrogance, and it's mostly Scottish woman who do this.
It's probably not arrogance, more likely just a phase they're grow with. Like the South has Ma'am.
Soldiers are almost always referred to as 'boys' in plural. If you address a group of males as "men" they usually will laugh.
At most I'd probably be slightly agitated if I was called a "boy" rather than angry. It's a semi-feminist thing some women in Britain do. They think they're better. But it's mostly harmless. Mostly Scots btw. British doesn't mean English btw. It involves all of the British Isles.