If you don't want an alternative to BT then order the line. Simples. Why are you here asking questions and coming over like a dodgy character trying to work a fiddle and present a false credibility.

They only levy a charge if it's due, and you messing about with the wiring probably will have no effect on that decision.

So if you have tunnel vision about using BT Business then get on with it and try your negotiating skills.