Ombudsman process is explained here
https://www.commsombudsman.org/our-process
Put a complaint to VM in writing
https://www.virginmedia.com/help/complaints
See if this past post from Cardiffman282 helps ref the online complaint form (although it looks like you need a VM login to use it)
Unable to make complaint RE: Offensive Offers | Virgin Media Community - 5618272
Raise the matter via email with the person (officer) shown at the bottom of this page
https://www.virginmedia.com/legal/privacy-policy
Raise the matter via email with the team shown under the power of attorney dropdown below
https://www.virginmedia.com/help/accessibility
Raise the matter with the ICO (suggest doing that by phone since time is not on your side)
https://ico.org.uk/make-a-complaint/data-protection-complaints/
One of the above may help, or none of the above may help, but some have had varying degrees of success in past topics through some of these routes (although admittedly VM's approach now does seem to be a flat 'no' to all requests for help in regaining access to mailboxes)
I am assuming that you are not a VM broadband customer (which may allow you some options) and it is a legacy virgin.net account you have lost