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).
Hope that's of use to you.