Yep unregistered, you are correct, straight line distance is not a guarantee.
However we've found several cases recently where people have moved house and taken a new phone contract with BT and been allocated a recycled phone number.
What has happened is that BT Wholesales database has not been updated with the new line length for the phone number, so when a user tries to sign up they are told they can't as line length is many km's.
When we spoke to BT wholesale they did a check and found the actual new line length was only a few hundred metres and so they update the database and everybody is happy.
So it's always worth querying if a line number check and a postcode check give vastly different speed estimates and you have just taken the phone number on.
Thanks
Admin