I live in an "unadopted" road and my BT pole is in my neighbours garden. I am in a cul-de-sac and what BT did (rather than digging to the pole near me) was to install a new pole at the top of the cul-de-sac so they could string fibre from the nearest pole on the main road to the pole in my neighbours garden.
Walking around my local area, it appears to me that they will bring the fibre up onto a pole which has a convenient duct nearby, then string it from pole to pole, installing additional poles if required (e.g. if they don't have line of sight to the next pole (because of trees or perhaps distance between poles etc.).
A few months before the new pole was installed, BT posted a notice on a lamp post near to the location of the new pole stating their intentions.
There are only 6 houses in my cul-de-sac, so I imagine it will be a few years before they recover the cost of installing that new pole