In situations such as this I've always done the internal installation of cabling and then runs cables (cat5e/6) to the front/side of the property and left them in a -78-p.jpg"]BT66 type box and then Openreach have connected on to them, fitted master socket at other end and then tested.
I normally opt to have the master sockets as close to the front of the property if not the other side of the wall from rising/dropping cable from BT network, this means that Openreach can changed the cable (if they wish) to there own and it won't impact any dectorating or add too much 'lesser quality' abling to the installation.
you have a long run of cable 'hidden' to the master socket an engineer may get a bit weary about using it, specially if its not higher grade cabling such as cat5/6.
Openreach will not use any 3rd party cabling or ducting external to the property, they have strict guidelines on that...
Also if your lines are fed overhead then consider terminating any cabling leading the master socket on upper level rather than lower level (minimise lengths and also external cabling)