My strategy is to see this as being in it for the long haul. Don't go in prepared to lose your head over it if you don't get instant gratification -- because you won't. I've waited 44 hours on a VM WhatsApp chat before I could get to a real person -- meanwhile having to tell a bot every few hours that, yes, I still want customer service to pay attention to me. I am now currently fending off the live customer service agent's attempts to haggle me into a new deal. (The trick is to use as few words as possible: "no. I'd like to cancel.") Still not quite there yet. It will happen, but it will happen on their time. Sort of like a war of attrition. Go into it with your head in that mode.
After this, based on several threads related to this subject matter, I'll probably have to deal with O2 separately, and the same thing will happen. The trick is to stick to your guns, keep it as absolutely simple as possible, and always go back to that letter which says:
If you're not happy with any of the changes announced in this email, including those to our out of home WiFi hotspots and pricing terms and conditions, you can change or cancel your package, including your Volt O₂ sim, any time before 4th April 2023, without paying any cancellation fees.