As for putting in my own duct it would depend on how easy it would be to liase with the installer, plus it will still cost for me to do it.
We put our own ducting across our land to meet up with the Openreach box in the verge. A lot cheaper than Openreach would have been and we could do it to our standard.
Turf cutter to take off turf, then mini digger for trench. Openreach supplied the duct, and we shoved it in the hole and then backfilled and relaid turf. 3-months later you couldn't see a single piece of where the trench was, and most of it was invisible in 4-weeks.
It depends on if this is the sort of thing you do (we have a tame digger driver) and the overall distance, the shorter it is the more likely that it would be easier to leave to Openreach. But you would be unlikely to be in control of fit & finish with Openreach.