Should age-off after six years, which I believe is the length of credit history files. That certainly won't be urgent enough for you, but there's nothing you can do to remove it - credit events can't just be erased, that's the whole point of credit checking.
If the debt has still not been paid, then you should ask VM if you can clear it and have your credit history updated to show that you have paid, but the record of a default will still be there. Even paying the debt will be difficult because VM's customer accounting have probably archived the financial data in a near inaccessible form, so there's no account or customer record to credit your payment to, moreover the debt will have either been bundled up with other debts and written off or sold on to a third party debt purchase agency. Not to mention that you'll struggle to speak with anybody at VM at the moment, and even then chances are they'll not have much clue about resolving this due to its age.
Sorry if that's not the answer you wanted, but that's how I think the land lies on aged debts.
My reply above assumes that the default did occur, and you are not disputing that. If it didn't and there's just some random default been incorrectly applied to your credit history file, then there ARE things you can do, although with lock down they may not be quick. If you dispute the default, then see the link below, particularly the bit on fixing and reporting errors on your credit file, but also take note of the opportunity to add a Notice of Correction to highlight extenuating or changed circumstances:
You could try texting Virgin Media to request a call back, on 07533051809, but I can't say how effective that will be - like all big communications companies, VM are simply atrocious at communication with customers. With response centres still suffering much reduced staffing due to lockdown and/or social distancing, I'd expect you'll have incredible difficulty getting through.
In the circumstances I'd write to the CEO of Virgin Media in Reading. He won't even see your letter, but his staff will be used to forwarding customer complaints and questions to the right people - but you still should not expect a quick turnround.