Having to use housebuilders' internet 4 years after moving in

unforeseen

Find out if there is some sort of right to manage of the service.

You'd have as much right to do that as managing your own electricity supply

It is shocking that they did not provide adequate infrastructure in the first place, you should contact the Planning Dept and ask them why they granted planning permission for these developers and ask they make sure it does not happen again.

They have provided the infrastructure though. Developers have the choice of selecting who they wish to provide it. They are not tied to Openreach. If the roads have been adopted then the option for another company to install their own infrastructure will be based purely on cost & ROI

ETA. I don't even think that the provision of phone/broadband is any sort of requirement of a planning application. There are small estates in my area that were built without it and are now stuck on mobile data until the roads get adopted and hopefully somebody will want to provion the service