Seems odd as I presume you can use a VOIP number from anywhere with an internet connection, or am I mistaken?
As with every fixed landline number, every contract mobile number and every registered PAYG mobile number, any given address is available to the emergency services.
You can use a mobile phone from anywhere with a mast but they still have access to your registered billing address.
If I make a late night 999 call from my mobile and get cut off halfway through the call a police officer well likely chap the door that mobile is registered to, regardless of where in the country the call was made.
Seems sensible to me, especially if every "landline" is going to be VOIP to continue having a fixed registered address linked to a number.
As for VOIP numbers not providing results on the BT Wholesale checker, there's still the address option.
I don't imagine it's their biggest priority.