I doubt the BT Statesman (connector) was compatible with the GPO socket in the kitchen hence the 2 way adapter which was taken away by Mr Pike with the cordless phone. Hence the GPO issued cream rotary dial phone was brought down from the bedroom.
Maybe June asked NB to put it back when they went to bed and it was overlooked? Why did June walk around the bed and back again if not an attempt to access the phone? June's mother was elderly and bedridden so she would probably want to be near a phone during the night.
Of course the GPO Statesman was compatible with the socket in the kitchen!
The reason Mr Pike took the socket with him was to test it in his workshop — are you suggesting he didn’t have any in his store where he repaired and sold telephones and equipment?

Yes, I’m sure June wanted to be near a phone in case her elderly mother needed her, and as June spent most time downstairs she’d have a phone down there, wouldn’t she? Which begs the question: why was the GPO Statesman phone hidden amongst a pile of magazines when it was in perfect working order and was the kitchen phone?
I agree both June and Nevill would want a bedside phone for emergencies, which is why Jeremy didn’t remove it prior to murdering his family. He had no need to — he simply disabled the phone line by removing the cradle off the cream Statesman phone in the kitchen.