bt don't charge £125 to just turn on the wire, they only charge installation if they have to run a wire from outside.
I was quoted £125 when I had perfectly good wiring in my house. In the end I got it for £30 as part of an 18 month contract deal, but the engineer had to do absolutely nothing in my house. They will definitely charge you if your line has no dial tone as that means it has then been physically disconnected at the cabinet.