I looked up your previous forum submission on this and wasn't 100% convinced that what you were getting was FTTP (rather than FTTC). Furthermore, if it were FTTC, you might not be getting a fast broadband connection at all, since you stated in your previous query that you're situated some 1.5 miles from the nearest cabinet. Normally, that would preclude FTTC, as your speed would probably then be under 15M bps.
I think that perhaps you need to find out more about the eventual new line from Openreach. As we've seen so often, just because a pole or a cabinet has been newly installed somewhere, it doesn't follow that a broadband service from that will be shortly available.
My view - and this comes from someone with a past professional engineering background in copper and optical fibre technologies - is that the line supplier - in this case, Openreach - will want, wherever possible, to put fibre into the ground, rather than aerially on poles. There are some good technical reasons for this. There are, of course, cost implications, either way, as well. But clearly if they're forced to distribute the optical signal to your locality entirely via poles, then your provision of a duct for the last few metres isn't going to make much difference in terms of its protection or the line performance; it might as well remain aerial, right up to it entering your house. And because you're 1.5 miles from the cabinet, this will apply also to any FTTC copper line for the copper leg. Incidentally, it's my understanding that Openreach wouldn't recommend you using an FTTC connection (as opposed to FTTP or remaining on ADSL/ADSL2) in the first place if you were that far from the cabinet. You may, however, have some other overriding wish to put the last few metres into an underground duct, such as a large tree or some other obstruction that would otherwise baulk an aerial route into your house.
Openreach may now have rules laid down about this but certainly some years ago, in the context of an ADSL line, I myself managed to persuade OR to replace a rather ropey and multi-repaired stretch of underground copper line in my own immediate neighbourhood and were happy for me to provide, across my front garden, an underground plastic duct, with suitable end cappings and allowance for water drainage, to get the new cable (the ropey existing one had been directly buried) from the pavement to my house. The said duct was around 4 metres in length and designed for burying. Frankly, there was little alternative, other than for OR to directly bury the cable in the ground, which would have made it vulnerable to being inadvertently dug up by me, my neighbour, or my successor during some gardening exploit. Openreach don't really like themselves messing about with people's properties/gardens, so were pleased to find a suitable duct in place, to get the cable finally from the pavement to the front wall of the house.