If you can not get the BT ones you want you could by any brand of DECT cordless phone and plug the base station into the Smarthub green phone socket and you would then be able to connect same brand cordless phones to the base station and place them where you want them.
OK EE got back to me saying they have rebranded the BT phones to EE phones and changed to offerings to what they call
EE Home Phone and EE Home Phone Plus
https://store.ee.co.uk/products/ee-home-phone-112937-HPRX.html
https://store.ee.co.uk/products/ee-home-phone-plus-112938-HPRZ.html
interestingly the Phone Plus looks the same as the BT essentials one but EE are charging £15 more per phone!
I see that the EE Home Phone Plus no longer has Alexa built in.
RIC9380