Hi Bambino. Still no luck, I'm afraid.
I got the following text a while ago: O2 Rewards: Sorry, we haven't been able to opt you in right now. Give us a call on 4445 to find out more. Calls cost 25p.
I think the problem may be that I was on O2 Sim-only contract customer several years ago and parts of the system haven't updated. Guess I'll have to wait till the lockdown starts to ease and I'll try calling.