Yeah, you can have it installed anywhere as long as the installer is able to run a coax cable feed to it from where the cables come into your building. They'd ordinarilly want to run the cables internally which can be messy, but some may run the cable on the outside of the house for you? I'd stress that you want this to be done when requesting an installation though so that the installer has the ladders required. In some instance Virgin will need to do a site visit to assess what is required if an installation is different to what they normally encounter. You may even be asked to pay extra if the installation is more complex than normal. Note that the V6 will also need an internet connection, but this can be facilitated via the box's inbuilt wifi capabilities.
There's no theoretical reason why you cannot have the V6 in your bedroom though.
Just to point out, I live in a large house, have 4 boxes (2 V6s and 2 Tivos) and there was never any question of running them internally, they ran them all externally to the nearest point to the TV (2 are upstairs - 1 at front of house, one at rear, 2 are downstairs again, 1 at front and 1 at rear). Mind you it's a detached house, I could probably forsee issues with a terraced house if they were at rear.