Yes the whole ground floor sockets circuit is wired in this stuff. Definitely large enough for a ring final, I assume it is an imperial multi stranded cable, I am not sure if it is 7 or 9 strands. What date did twin / 3 core with no earth end? I thought this house was about 1960's / 70's so would have been too recent.
I once encountered a ring final wired with 1.5 3 core and earth, using yellow as earth and CPC not used. I put that one down to the apprentice first fixed, picked up the wrong drum of cable (very similar size and weight to 2.5 t&e) and it was an "oh sh!t what are we going to do" moment when they went to second fix.