You're missing the point.
All cordless phones obviously need a power source to charge them or continuously have non rechargeable batteries changed.
The adaptor is simply a portable phone socket and is only required to connect a corded phone to the hub.
Any cordless (DECT) phone can be connected to the hub.
Edit: a quick Google search reveals DV handsets can be obtained from Amazon for £32.99
BT did initially supply a DV cordless DECT phone for early adopters of DV , but that turned out to be something of a rod for their own back , no credit for supplying it in the first place , and criticism for when the battery died, or didn’t meet the customers expectations…chances are the reason they are hard to locate is BT/EE eventually realised it was a stupid decision to supply them and stopped their manufacture changing to the much simpler DV adapter.
Consider if you were a Sky , Vodafone or Talk Talk customer (and many others ) , they also would provide their telephony service via the their router but supply no equipment for you to use , you woukd be on your own as far as that were concerned.
It’s now been said multiple times , a DV phone is just a DECT cordless phone , it clearly needs mains power to keep its battery charged via its base unit or cradle if it’s a multi phone system.
A DV phone has a single button to listen to any messages left on the 1571 answering service , a non DV phone would need to dial 1571 to do the same , that’s about the extent of the advantage a DV specific DECT cordless phone has over a regular DECT cordless phone , but importantly, they all work with DV
I take your point - and I know it works as my main phone, by the way it is a DECT BT8600 (moved from kitchen to router) still connects to the one upstairs, which is just plugged into a mains socket.
I saw the so called new phones advertised and even now on the EE website and thought - that's the way to go but couldn't find them. I know sometimes if you use EE/BT products, any issues and they can't blame another provider.
I will just order the BT premium 3 handsets and go that way! (unless someone think another phone is better)?
So just for clarification. I buy these three, one of the phones - preferably the main phone, will have to be plugged in via the adapter (if not in the router), the other two will just connect using DECT tehcnology when register the handsets (as the BT8600 now)?
If you don't want to plug the base station into the hub, then yes, plug it into a conveniented located adapter which will be exactly the same as plugging it into your old phone socket.
Alternatively, just register the handsets directly to the hub so you don't need an adaptor at all.
There is no concept of 'main' phone, they are 3 handsets connected to a DECT base station. Your upstairs phone is just sitting in a charging cradle.
As already posted, DV handsets are available from Amazon.