What Merula says is true sometimes. Some counties are reticent at upgrading cabinets if there is even partial VM coverage (faster shire springs to mind), while others will consider upgrades even with substantial coverage (Warwickshire springs to mind).
However, the biggest hurdle in phase 1 is that the NGA white areas (the intervention area) are very set in stone. Postcodes that have since been built are SOL, unfortunately.
In phase 2, the most open counties are including rules that let them change area. A lot of projects are going through phase 2 procurement right now, so at minimum, you want to make sure your postcode is defined white for phase 2.
I wrote a lot for someone else over on this thread, which may or may not apply