It's called lock-in
"There are technical reasons relating to the bespoke and complex nature of the solution which would lead to substantial duplication of costs and unacceptable technical risks which would not allow for the service to be transferred to another supplier "
As the saying goes : when you've got them by the balls, their hearts and minds will soon follow. So it has happened here.
That said, I'm in a bit of a bind here ; as much as I'd like to make a scathing remark on how BT is screwing its customer, this is a telecomms project. I do not know that there are any Open Source telecomms infrastructures available, so I guess that BT or anyone else would have landed the NLPS in pretty much the same boat.
Is there any other provider available in Northern Ireland ?