Downgrade online is not possible
This page, https://www.virginmedia.com/help/broadband-tv-and-landline specifically states Whether you’re thinking about changing your broadband package, TV or phone service, or changing your Virgin ...
Forum Discussion
VM wants you to phone up and engage with a sales agent who will try to upsell you a lot of stuff you probably don't want.
The 'bundle' approach for discounted packages has been used by VM for a long time. It can work well for those who reliably phone up every 18 months and haggle a renewal.
You haven't said what service(s) you do want in the future. You should be pricing what the rivals to VM can offer before you engage in negotiations with VM.
The phone switchover programme was put on hold, for all telecoms providers, by the previous government. I think I read on here that VM has restarted it on a smaller/slower scale than before. VM continues to switch customers with a faulty conventional line as an alternative to making a repair.