You said you don't have to give any notice as you're on Pay & Go. I mentioned the Community Reward because if you were intending to give notice to O2, which you wouldn't, you would lose that perk. I'm not going anywhere with it. That's just a fact.
You have mentioned in numerous threads that you can't get TuGo because you're Pay & Go. If you were on a 30 day contract you could have it. The IOS calling over WiFi feature isn't free. It still uses your minutes and texts. If you had a 30 day contract, many of them are unlimited for calls and texts.
I don't think I know anyone who has just cancelled a contract and not gone somewhere else. Do you?
Yes, you get a final bill if you're on contract, but if you use the PAC correctly and in a timely fashion, there is either little or no money to pay, or as it was in my case, you get a refund.