As the developer did offer the work to be done as an option and you declined that option, it would be difficult to complain to the developer.
Your option now is to get in a contractor yourself to do the work (you will of course need to pay this yourself). You might have thought that what the developer was asking an extortionate amount, but you might find in the long run that it would have been a cheaper option to have paid the developer to do the work prior to the property being finished.
you need to remove the amazon link as not permitted on forum - or mods will
For clarity, the responsibility for any wiring within your house is purely between you and the developer it has absolutely nothing to do with BT or Openreach. Openreach purely provide a suggested scheme that developers may or may not wish to implement. All Openreach provide is an ONT with one Ethernet port that connects to the WAN port of your ISP provided router, if that ISP is BT, they provide a Home Hub.
It is only a recommendation. All the requirements the builder must meet end at the ONT as that is all Openreach take reponsibility for.
It is very short sighted on the developers part though. They are responsible for all internal wiring.