There are very few new ducts used for fibre. BT will, largely, have been using the existing E-side ducts (carrying exchange<->PCP copper) to feed the fibre down. These will have been laid for copper a long time ago; new ones are only needed where the old ones are full.
That's the obvious routing for all the exchanges that are "Parent" ones.
For child exchanges, there are a couple of options:
a) The fibre arrives in the manhole just outside the "Child" exchange, after following trunk ducting from the parent, and then starts to follow the various E-side ducts of the child exchange. The fibre itself gets right up to the child exchange, but doesn't go in.
b) The fibre arrives by completely different routing, as per Kitz' picture. That fibre might come out of the parent following existing E-side ducts, before needing a short length of new duct as it hops over the existing exchange boundary to the first cabinet in the child exchange.
When the new FTTC cabinet gets built, there is normally some short lengths of new ducting, to join the cabinet to an underground chamber, and to allow the copper tie pairs to route from the PCP (via one or two chambers) into the FTTC cabinet. The fibre will also use one of these short lengths of new duct.
A recent presentation from BT showed there are over 4,500 exchanges with at least one cabinet which is fibre-enabled. 1,500 of these only have 20CN exchange facilities, so certainly don't have a fibre head-end. It wouldn't surprise me if there were less than 1,000 head-end exchanges. Perhaps even fewer.
