Read the T&C's available about VOLT on VM's web site. It's black and white, if you cancel the O2 SIM within the cool off period though, you lose the speed and other VOLT benefits (this is a change from original T&C's for VOLT as VM tightened up this area). As it seems you got your VOLT through a VM bundled package, the contract basically says if you cancel the O2 SIM completely within 14 days you lose your speed.
If you were volted because you already an O2 SIM separately and that contract was either not in a minimum term, or the minimum term was ending soon, you would have kept your speed until contract renewal on the VM side. This does not apply to you as you got the O2 SIM as part of a negotiated VM bundle deal.
You used to be able to go to uswitch.com and "upgrade" to the cheapest O2 SIM - it restarted a 12 month contract on the O2 side, but was say for £6 per month and was still VOLT eligible. I'm not 100% whether VMO2 ended this as you used to sign in with your O2 account during the uswitch checkout process (which was O2's checkout) and selected upgrade. It started warning about early termination fees, then that got removed when you went to final page of the checkout. The advice was there to scare people off, but in reality, O2 always marketed the tariffs as change at anytime up or down, so it was dodgy ground to try and enforce.
Contracts from April 2025 T&C's:
https://a.storyblok.com/f/253875/x/4220195753/terms-and-conditions-tv-fibre-phone-from-24-apr-2025.pdf
"If you subsequently cancel your eligible O2 Pay Monthly Mobile Plan,
where you are the registered account holder of it, or it is cancelled by
the relevant account holder after the 14 day period referred to in
paragraph A.5 above, the Volt benefits for your Virgin Media services
may be lost when you next recontract to Virgin Media’s services."