Unfortunately for you VM's email service is supposed to be discontinued 90 days after an account is closed, although due to VM bungling this closure many customers have (like you) used it free for years after leaving. Some providers (notably Sky) keep email accounts open indefinitely, but VM never intentionally did. Seems likely that they've caught up with you, and I'm guessing the account has been closed and deleted and in that case you've got any emails saved on local machines, but anything on VM's servers is now irretrievably gone.
What can you do? Nothing, I think. VM won't raise a ticket for a expensive non-standard service request to recover the deleted account of a former customer. They probably could if (eg) the police or security services requested it, but for you or me, nope. You allude to paying £20k to VM for services previously, but that was exactly that, services previously rendered, not as payment for the enduring provision of an email service that if you'd bought as a standalone service would be about £40 a year.
Your legal rights here are nil - there's no contract between you or VM, so neither contract law or consumer rights offer any protection. As a technicality, accessing VM's email servers when you aren't a paying customer would potentially qualify as an offence of unauthorised access under Section 1 of the Computer Misuse Act 1990. VM have also committed an offence of rather greater severity, in that they breached GDPR by retaining and processing your data after the contract ended (although you continuing to use and access the data could easily be read as continued permission, albeit without creating an obligation on VM).
In practical terms VM aren't going to prosecute you, and I reason you'll not be taking action against them but the failing of both parties does help emphasise how you really have zero rights in this situation. I appreciate that's not what you want to hear, but that's how it is. It also illustrates that if you aren't paying for a service, then you have no rights. Think about that if using any email service for which you're not paying, such as Gmail or Outlook type freebies.