UK is under BST (UTC+1) for half the year but people are usually just taught that the UK is GMT (UTC+0) which is based in the time in Greenwich, withought mentioning DST. I suppose it's also possible everyone is taught BST and just forgets about it because daylight savings sucks, but either way most people seem to think GMT and UK time are the same thing.

This means you'll get people asking for GMT times when they want BST or UK local time.

I too was similarly confused by the original comment at first, but I think they're referring to the fact that 6pm GMT is 7pm in London during the summer (BST), and 6pm in London the rest of the year. It seems OP and "them" are both correct in that hypothetical exchange.

The term UT is specifically created to divorce universal timekeeping from whatever UK's timezone is doing, whether DST is on or not, etc. and the derived term UTC for when started adding leap seconds.

GMT hasn't been a thing since UTC. It should be scrubbed from the vocabulary and old farts still using it should be shunned and reeducated.