I've been trying for years (by example) to popularize using PT, MT, CT, ET. Since it's always after a number ("Are you avail at 4ET?") there has never been one instance of confusion, and I save typing and avoid the small but real possibility of being interpreted too literally (suppose someone asks a computer "What time is 4:00 PM EST in London time" during the summer, and gets a 'technically correct' answer).

I don't have a similar answer for GMT/BST though.

Then they are wrong.

I also only use the S variant in human conversation in my time zone.