These are my findings and no way should be taken as gospel:
The V+ box seems to have it's menus (EPG) written for 720p. Using 1080i seems to blur the menu system so just stick it on 720p for now. Hopefully Virgin will release a new software version for the box that will use 1080i without the blurring.
But, 1080i does look better on the BBC HD channel (unless you are watching fast moving stuff).