Wasn't it once allowed in the distant past, a single extra spur, from a socket which was already a spur?
Non of my regulations allow it, however in the days of 7/0.029 it was 2.9 mm² and a fuse was 30 amp, so that would be same as around 26 amp with 2.5 mm² which is over the rating for the cable in most cases, but not to same extent, also there are cable types, even with plastic with have thermal plastic and thermal setting cable, thermal plastic rated normally at 70°C and thermal setting at 90°C in the main with twin and earth the latter was white sheaf with former grey but not all the time, so with 7/0.029 thermal setting it may have been allowed and with 7/0.036 at 4.52 mm² it would not really have been a problem.
However even on the ring final 2.5 mm² can be over loaded near the origin, and once you take a spur from a spur you as said can in the future think it is a radial and extend, well it is of course a radial once more than one socket is fitted, so if you want more that one socket it should have a 13 amp fuse.
As to complying with a previous edition this is hard, as in the main we don't have that previous edition to refer to, so shower in a bedroom in days of 16th edition can you now find the rules? I still remember the rules, and in essence they have not changed, but it stated no socket within 3 meters of shower, but 14th edition I have an old guide, and before that only hear say, so pre 1966 no earths to lights, and a distance for socket to a sink, which I am told was never removed from plumbing regulations.
Dads house many spurs from spurs, seems 1954 when built it had 6 sockets on the ring final compared with granddads house with two that was good, but it was extended and extended and when all removed for re-wire did find some clearly stressed cables all were 2.5 mm, non of the 7/0.029 showed any signs of being overloaded.
