That's strange to me that you mention London as being similar to Glasgow. I've not had the chance to visit either, but my go-to on this topic has always been this faux news story on the Mash Report:

Northener Terrifies Londoners By Saying "Hello"

https://www.youtube.com/watch?v=1YxLiLFjYKc

I hadn't noticed it before, but they even specifically mention the use of headphones as a defense mechanism near the end.

Yeah the most unbelievable part of this is people randomly chatting in London.

I live in London, and I remember flying to Ireland at some point, and I was seated next to an Irish lady returning home from holiday who sparked up a conversation.

My initial reaction was "Why the fuck are you talking to me?" because I had lived in London so long I was thrown off by a random person sparking conversation. But turns out she was just a lovely Irish lady flying from from holiday

Maybe it depends on where you are and the cultural makeup of the part of London you're in, then?

As an aside, I used to work for the company in Glasgow that build The Daily Mash's website before they got all into the ad-heavy money-at-all-costs thing. It used to be much better and the guys behind are actually pretty decent.