Most VOIP providers can do this "out of the box" with their hosted VOIP solutions, and can easily port the numbers from BT.
I suggest you take a look at voipfone.co.uk or any of the other members of the ITSPA alliance.
You will of course be dependant upon your internet connection at the new premesis for your voice services, and some of the VOIP providers can offer their own branded ADSL with the necessary Quality of Service guarantees to ensure it works. And a lot of them also provide divert on failure service options to mobiles if your VOIP handset is uncontactable.
Hope that helps
