Can't say I am aware of any VM features or utilities to get this info.
There is a section in the DSAR request form for 'service usage data'
https://www.virginmedia.com/help/dsar
but it's not clear what time period it covers (may be just 3 months)
In the past, VM used to send out marketing info showing alleged usage data on the account (which I have received myself in the past) but the info it contained was wildly inaccurate and random compared to anything I might realistically have ever used. So, even if you do find a way to get info from VM, it may not reflect your real usage in any way at all.
Why do you need to know your data usage in order to switch broadband provider?