I've been part of one UK and several Australian congos (with many UK people), and visited a couple more UK congos and a couple of US ones. You live there and have lived this experience, so you know better than I. But you've raised a general point too.
As I've expereinced it, there are many British cultures where being sociable, or 'agreeable' is an important thing.
That, I think, comes out in the STYLE of enforcment rather than in the actuality - there's a thing about subtley ('hinting') and not upsetting others in some parts of Britian that seem 'nice', but in my limited experience covers a heart of iron lurking just under the surface.
Being 'unevenly yoked' is not only a rule, but part of the social outlook of JW's. If your wife was an active JW in good standing who and was known to have started a relationship with a worldly person, then it's inconcievable she wouldn't have been counciled and been deemed 'spiritually weak' irregardless of where she lived.
If she was 'on the outer' at the time they may have felt she was not then part of the congregation which is a reason for taking limited action.
The style might well have been 'nice' enough not to have traumatised your wife, and that's your good luck. Maybe she didn't care, maybe she didn't notice, maybe she didn't tell you ... but if it was known, the JW's did treat her differently.
Once you were married and moved on, it's a different story. She was almost certainly be limited in what 'privilages' she'd be alowed to partake in, at least for a period of a few years. For example, if you tell me that she was approved as a Regular Pioneer in the same year she married you, I could not beleive that to be accurate.
As time goes on, the issue will loose currancy - regrdless of where you live - becuase you're married.
They're unlikley to stop your friend's GF becoming a JW, and they will probbaly be polite and friendly to her BF. If they marry, things will settle over time.
But in the meantime she will be treated differently, she will be restricted in what she can do and where she can go (both 'spiritually' and socially) and the motive will be to prevent her being 'unevenly yoked'.
The local elders might be nice about it, or they might be nasty. But they will act that way.
If they don't, let us know and we'll dob them into Mill Hill as likley future apostates... :)
Max