As far as I understood it, if you already had an O2 account with any existing Pay Monthly plans, then it's that account that can be 'Supercharged' and linked to your cable services via O2 (either My O2 or via O2 Customer Services). You shouldn't need to take out another O2 SIM to get a Volt bundle deal with Virgin Media.
I'd suggest you speak to O2 Customer Services (0344 809 0202) to work out what can now be done - it's possible they can cancel the new SIM and link your existing SIM. That would avoid any PAC or EDFs.
There is a 14 day cancellation period for new SIM sales on O2
https://www.o2.co.uk/help/device-and-sim-support/returning-your-device
Can I return my sim only contract?
If you’ve taken out a sim only contract in store, over the phone or online, you can change your mind within 14 days of receiving your order. If you do want to cancel your agreement, contact us to let us know and we’ll get it sorted. We won’t ask you to return your sim, all you need to do is cut the sim in half once it’s stopped working.
We’ll give you back all the payments you’ve made, apart from any buy-out fee from your previous contract or any express delivery charges. You also have to pay any usage charges you’ve had in that time.
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I work for Virgin Media - but all opinions posted here are my own