It's very hard to tell from the Talktalk blurb whether I'm getting G.FAST or what. I'm currently on their Fibre65 package with an old fashioned master socket just having a flat telephone socket, and I'm supposed to be moving to their Fibre150 with a master socket having both flat telephone and RJ45 sockets. It's all delivered over a single twisted pair. There's no question of fibre coming any closer than the street cabinet a couple of hundred yards away.
From that description it sounds very much like G.Fast; Talktalk's product naming scheme is very confusing.
Your new master socket is not technically RJ45, and you definitely cannot plug ethernet into it, but it's a similar connector (RJ11? 6P4C? I'm not sure of the exact spec)
It still only carries the one copper pair with the broadband signal, but the faceplate has a built-in filter, to separate the broadband signal from your extension wiring.
So my key question remains: when POTS-switchoff happens and I get sent a VOIP-capable router, will I still be able to connect it via the flat telephone socket and my existing extension wiring I installed 30 years ago, or will I be forced to connect it via the RJ45 socket, requiring some rearrangement on my part?
The latter. The same is ideally the case for VDSL - you *should* connect your router directly at the master socket, because performance suffers if you hang it off a microfilter on an extension socket. It's often acceptable for VDSL, especially since you're so close to the cabinet, but G.Fast is *very* sensitive to wiring issues.
When POTS is turned off, your broadband becomes SOGEA or SOGfast (SO = "standalone"). The voice service, if your ISP continues to provide it, will be from a port on the router. It's your (powered) router which provides the analogue voice signal and the line voltage to drive a phone - instead of the exchange.
So if you want to use your existing extension wiring, you'll need to make a separate copper connection *back* from your router's POTS port to the master socket - or to any other convenient extension socket where it can connect to the extension wiring.