Shame about the fact that there is no working analogue wiring beyond the BT master socket, would have made life easier.
I believe what you are looking for is an ATA (Analogue Telephone Adapter) to add to your setup and most current VoIP providers will port your existing landline number over when you decide to switch.
Brief:
An analog telephone adapter (ATA) or FXS gateway, is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a voice over IP telephone network.
Then you can implement for simplicity a number of power-line adapter to provide connectivity between main router and each individual adapter / analogue telephone.
Ideal ATA adaptor for each analogue phone is something like the Grandstream HT801 ATA Adapter for around £40 each .
Unsure if there is a wireless ATA adaptor alternative but maybe worth looking if this is your preference.
Hope this helps as this a a tad more complicated than either I would ideally use 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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