Agreed- a) we are dealing with a different set of regulations and b) whilst the OP may be a very capable chap, the fact that the answer to this question was not abundantly clear when first setting eyes on the illustration, suggests not enough experience to safely tackle a CU installation.
Re upgrading to SP+N, the supply may previously have been a small TP+N service, which was once common on the continent, where the total capacity was adequate but no one phase could carry any large loads so cookers etc all needed to be three-phase too, which made things complicated and limited diversity,
Re upgrading to SP+N, the supply may previously have been a small TP+N service, which was once common on the continent, where the total capacity was adequate but no one phase could carry any large loads so cookers etc all needed to be three-phase too, which made things complicated and limited diversity,