Hi Sean,
New VM customers on VM's Full Fibre (XGS-PON) network can have up to 6 Stream (Flex) boxes on the Mega TV package. These are non recordable boxes streamed over the internet.
As far as I'm aware new customers on the older cable network receive one TV360 box and any additional boxes would be Stream (Flex) boxes - it might be that they can have one TV360 box and up to an additional 5 Stream (Flex) boxes if taking the Mega TV package.
The above are new customer scenarios, so I don't think it's possible for current customers (even by regrading/cancelling their current TV service) to take advantage of those multi-room capabilities
However if you aren't bothered about only having one recordable TV set top box then maybe it's possible to cancel your all current services and then you (or someone else in your house) sign up as a new customer to take advantage of 'up to 6 boxes'?
EDIT The following page shows VM's multiroom policy which is still restricted to either three TV360 boxes, or up to six tivo/V6 boxes of which no more than three can be V6's. ( it doesn't state anything about the Flex (Stream) boxes)
https://www.virginmedia.com/tv/multiroom