The checker states the following when you switch to the address checker:

"Warning
Please note that the address checker is less accurate than the telephone number checker for line-based services (e.g. FTTC) because the result is not based on an individual telephone line at the premises.
For premise-based services (e.g. FTTP) the address checker will provide the most accurate result. Note that BT's ability to provide Broadband to your address is dependent on several factors, including the availability of broadband at your serving exchange, and network connectivity for the various product options."

So I'd always recommend the address checker for FTTP checks.

Which is kind of obvious, when you realise that FTTP lines aren't associated with any Openreach-provided phone service (apart from a tiny number of legacy users on FVA).

I would do the install FTTP first and then port the number to VOIP to cancel the FTTC, but first I would check with Plusnet that they would not charge to end of contracted term as they could see it as not being a transfer.

wow you guys are quick, I realised I'd put talk talk when I meant EE, and quickly change it....but not quick enough.

Anyhow, plusnet support personel have confirmed on their forums that they will waive termination fees for fttp installation by BT/EE. It's not an automated thing and a bit clunky, but at least its an option. Clearly something to do with the fact that plusnet are part of the same group.