How attached are you, to your mobile number?
If you're not bothered, then simply stop using your V-Mobile SIM and take out a new line with EE as required. It would be more complicated if you want to retain your number as Virgin will need to allow a PAC.
Disclaimer: I'm not familiar with the Virgin Mobile side of things, but my career background is in mobile comms, and you've unfortunately fallen foul of one of the inherent problems with an MVNO. If their host network changes, and you were reliant on the previous host network, there's nothing the MVNO can do about it.
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