Issues with WiFi coverage from the VM routers, and very similar stories to yours, have been doing the rounds for years and years - nobody has ever gotten to the bottom of why this should be, as the WiFi specs are fairly standard - speculation that it’s poor design of the hubs and the antennae placement or maybe just a bad implementation of the firmware driving them, have been mooted - but nothing definitive.
Ultimately it is what it is, now VM do, sort of acknowledge this by offering their WiFi ‘pods’ and a sort of ‘guarantee’ of speed throughout the house. But this guarantee is a bit vague, ie do you get a consistent 20 Mb/s in every room, and there are requirement about what a ‘room’ is - not 100% sure a loft conversion would count! Even if you can show this, then VM will send you up to three of their ‘pods’ (and believe you me, the hoops you will need to jump through to get those), and if that doesn’t work, they will give you a one-off, £100 credit and wash their hands of the whole thing!
On the other hand, the VM hub does allow you to put it into bridge mode, which disables the WiFi entirely along with the routing functions and lets you use you own equipment - do you still have the Ubiquity kit? It’s certainly something to consider.
