This confusion is made more prevalent by Windows, which has consistently claimed the UK is in some variant of "GMT", or even "UTC", during summer, often with a timezone name like "GMT Summer Time" (the UK uses BST in summer). For example, right now, the Windows 10 timezone selector is claiming London is at UTC+00:00, even though it's at UTC+01:00 for summer, and earlier versions of Windows would claim it's in GMT.

This wouldn't fix anything, because people would still have to convert UTC to their local time before agreeing to anything, and they'd make the same mistakes in that process ("Shit, are we +7 or +8 right now?")

Not so much, because they'd be more aware of their UTC offset. I usually get people not even knowing that their timezone changes with DST.

The key here is the Brit said GMT aka UTC your the one that messed up here

No, the Brit thought he was scheduling it for 3pm on his wall clock, because he thought "GMT means the time we use here in London" but London was actually using BST that day. Source: have seen this from many many coworkers who should know better.

Yep. As well as a bunch of "10 AM PST" when they meant PDT. Nowadays I have to correct everyone as to their timezone.

If they use PST in the summer you can be pretty sure they mean PDT, at least, especially if you are somewhere where the abbreviation is unambiguously US Pacific Standard Time. With GMT/UTC there's some chance they were trying to express the time in a "standardized" way.

Yes, if you're already familiar with PDT... Not so much if someone says "X AM PST" and you just plug it into your calendar.