I’ve verified that the safe sender procedure works (but only from a web connection to BT Mail - apps like Outlook or iOS Mail don’t support setting changes). I’ve sent instructions (from a BT account) to gmail blocked club members.
I’ve also set up a distribution list of blocked recipients (ironically on my BT email account) that I’ll use to duplicate blocked gmails until the undeliverable messages stop.
If anyone from BT reads this - I’m not impressed.