I've struck solid 2.5mm here a few times in houses built or re-wired during the 60's.
Metric size cables were introduced here around 1970.
Here are the T&E sizes which were used before:
https://www.electrical-contractor.net/pc/IEE1966_T3.JPGNote that all were stranded, except for the smallest size. Ring circuits, for example, were typically wired with 7/.029 cable.