BS EN 60073:2002. Basic and safety principles for man-machine interface, marking and identification. Coding principles for indicators and actuators. There is lots of missing information, I will give you that, but … IF the rings are wired in 2.5conductors and 1.5CPC then IF the overload protective device [OPD] is 32A or less, then the cables should not overload so not even a C3 as you can only give a C3 where a regulation has not been met, not for something that doesn't comply with your method or working or your opinion. Perhaps the most important for labelling on equipment, in addition to the requirements of BS 7671 are:

Why a C1 and what regulation states you CANNOT connect two rings to a single overload protective device? A C1 is only warranted where immediate danger is present, typically but not limited to exposed live parts.BS EN IEC/IEEE 82079-1:2020. Preparation of information for use (instructions for use) of products. Principles and general requirements p>It is NOT a C3, and unfortunately the NIC have not asked essential questions.

For example, What is the rating of the single fuse that the 2 ring mains are connected to? 

If its a 50 A fuse then either ring main could draw 50A on 2.5 cables - or even more with short time overloads that the fuse will ignore. And thats IF the rings are wired in 2.5, which was not asked either!

Such a high load current will overload the cables and can lead to overheating and worse!

IT IS A C1 because there should not be 2 ring mains connected to a single fuse - EVER, and that type of installation practice would not comply with any previous edition of the wiring regulations, so we are not talking about coding it C1 to upgrade to the latest regulations

We are talking about coding it to correct non compliant practice that carries a potential fire risk.

The only conclusion that can be reached is C1

Always prepared to accept I'm wrong, but without trawling through the regs, I cannot think of where it says you CANNOT connect two rings to a final circuit, you can group circuits, so unless grouping circuits specifically EXCLUDES ring circuits, then there may well be no issue at all with this particular situation, once ALL the pertinent information is made available. BS ISO 3864-1:2011. Graphical symbols. Safety colours and safety signs. Design principles for safety signs and safety markings BS ISO 3864-4:2011. Graphical symbols. Safety colours and safety signs. Colorimetric and photometric properties of safety sign materialsNo, completely avoiding filling out anything at all on three-phase commercial final DBOs so no-one has a clue what feeds (or isolates) what ! Not sure any spark worth using would fit two rings on a single OPD, but you tell me the regulation number where it says it can't be done and I'll allow a C3, or a C2 providing that the OPD is rated too high for the cable in use.