We likewise had success by getting together and working as a group, though we managed to prove commercial viability for a group of cabinets in an area full of other enabled cabs with high take up, and did not need to resort to gap funding.
We started by setting up a website (link below!) and a couple of us leafleted all properties connected to the cabinet - around 400 - as well as promoting the campaign on social media and the local community magazine. Over 100 people responded stating they would take out a service in the first 6 months.
We did some research to find contacts at BT/Openreach, but also crucially got the support of our local councillors. What also helped was then also getting the campaign extended to three cabinets all on new build sites suffering the same fate. We and our councillors then contacted the local BT Regional Partnership Director (details on BT website), and believe other residents also sent them the odd direct email! BT, and then the regional OR programme team, started speaking to us, and then came out for a meeting with us, which we prepared a 'sales pitch' presentation for.
Obviously if you can't prove commercial viability, you're probably going to need to resort to gap funding - if you approach OR about that, it should at least be easier to get a response.
As per the last post, try to also get everyone in the area registered on Virgin's Cable My Street site (and the Superfast Openreach site), as I'm sure that helps too. Again a local campaign/leafleting helps. Good luck