If you cannot verify the SWA is earthed and its inaccessible, C 2 all day long on a Sub main cable, its most likely being feed from a ryefield board of some sort on a 63 amp Fuse,
I kind of get the consumer unit end but at the Electrical cupboard, that's poor.
you also have know idea of the cable route it take's or how many other flats it could travel through buried within the wall possible not even in safe zones just to get to the flat it serves.
C2 and unsatisfactory . cover your self and hopefully get some work out of it.
Just out of curiosity why would you give a non RCD'd circuit covering a Twin and earth buried cable as a C3?
is this not potential dangerous ?