A friend (not me - honest, so the details may be inaccurate) had a similar problem. There was a phone socket in his new abode, but the line was not active - i.e. no dial tone. He used a "3" dongle to sign up with Plusnet. Broadband was activated within 2 weeks. Two weeks later the phone service was activated.
In your case, you should be able to reduce the 2 week activation period by starting the process now.
As previously mentioned, the details may be inaccurate, but I don't recall any whinges about an exorbitant fee for line activation.