I know this is an old thread, but it's the first one that comes up in Google, and more and more people will be looking for the answer.
BT won't ever use Voneus infrastructure. The business model for Voneus is to do the minimum possible to get fibre installed, and then take the government grants and run. Once a customer has signed up, Voneus are no longer interested in them, and it doesn't matter if the customer leaves - the money for Voneus comes from the installation, not from the ongoing contract. It is a short term strategy. Their infrastructure cannot last any length of time, and will need repairs within a few years. (Look at the depth of some of their cables. It's about 100mm underground. A few lorries driving over it will be enough to destroy it.)
BT Openreach have a long-term business model. They want to be in business in 50 years' time, so they perform, and rely on, proper installations. If BT used the infrastructure of Voneus, their service would be plagued by disconnections and angry customers.
BT Openreach is equivalent to National Highways. Voneus is equivalent to a drunk DIYer who found a wheelbarrow of tarmac.
I doubt BT Openreach will install a parallel service to that of Voneus. Instead, you will be trapped with the option of Voneus until the complaints about Voneus get too high, Voneus go bust, and then some other company buys up the remains. Meanwhile, the government will boast that 100% of the population have "access" to fibre, even if some of that fibre is unusable.
