Despite the assurances from forum staff, we've seen instances where the postcode checker says "yes", and when the order is placed VM say "no". It is generally accurate, but if you're making big decisions about buying or renting property I still say you need to have a much stronger assurance before committing to buy or rent.
Fundamentally VM's checker is based on the Post Office postcode address file (PAF), mapped onto VM's service area, and then with an overlay to rule out known non-serviceable addresses or areas where due to congestion they're not taking new connections temporarily. Unfortunately this is still a big generalisation, and at a default level assumes a postcode is serviceable until proven otherwise. Especially for flats there's potential for multiple complications - wayleaves for cables across private property, letters of permission if there's a service company, cable routing and access challenges if using service ducting, potential for connection costs that are easily outside the limited budget for individual installs. We've even see instances where some flats are already served, and VM refuse to connect others in the same block. I've worked for large businesses with infrastructure assets, and no matter what the GIS database says, no matter how proudly the company announce it's unerring accuracy, there's always instances where the GIS is completely wrong. And in this case, we're not even talking about a proper GIS, just a cobbled add-on to PAF.
If staff can give you assurance that you'll trust that's good, but bear in mind if the company later on turn round and say "no", then you'll be stuck with a flat that has a slower connection than you want, and your only recourse against VM will be formal complaint + adjudication, and probably small claims court action.