My son finally had his FTTP installation completed last month after 3 unsucessful attempts at monthly intervals in running the fibre from adjacent chamber directly to the existing POTS/BB master socket in the hall, in this case the existing POTS cable was ducted.
One repair was made on a collapsed duct in the neighbours drive and the fibre made it to within a few feet of the master socket box (concrete floors) but would go no further.
Luckily the contractors, KN Circet, on their final visit persevered and dug down to the duct outside the front door, extracted the fibre and ran this along the bottom of the gable wall into the under stairs cupboard, quite a neat job in the end but did take 3 months from initial placing of the order.
My own situation is very similar to yours, UG feeds, area just being ducted for fibre at present, POTS distribution chamber just outside my front garden wall and fairly sure my existing POTS cable is directly buried across the garden but have not been able to look into the dist. chamber yet, will likely get a peek when the contractors start to pull the fibre this week.
I do know the power and POTS cable enter the house in the same duct, concrete floors, I remember the BT supervisor who I knew at the time ranting at the building site foreman, over 35 years ago now, I believe the ducting only extends to the front edge of the house.
As per yourself, I woud like to try and make some provision for a neat fibre transit from the dist. chamber to a house entry point of my choice but not sure as yet how this would be best accomplished.