The main point is 2.5 mm² cable is often down to 20 amp rating depending on installation method, so we don't want to over load it, second point is it is called a ring final, the word final is important, there are other types of ring, but it is designed to supply multi 13 amp outlets, the ring final can be up to 106 meters of cable, over that there is too much volt drop, but will so much cable if you get heavy current draw at one end that end could be overloaded.
So a 4 mm² cable at centre of ring supplying over 13 amp would not cause a problem, but the same ¼ way around may cause overloading, also two sockets on a spur is clearly the same as one double socket, however knowing a second socket has been added or that this point is ½ way around cable length wise needs expensive test gear and good plans, and these are often not available.
So the rules say either one socket be it double or single, or one fused connection unit on any spur, this way if you see two cables you know part of ring, and it is unlikely with 13 amp limit with a FCU you will overload any part of the ring.
So my man cave has a consumer unit with 14 RCBO's and it feeds the whole home from there, but when I am ready to add out buildings they will come from a 16 amp RCBO to a single back box where the twin and earth becomes SWA. And it will not connect to any other home circuit, one reason is I don't want any fault in out buildings to effect the house, but the main reason is who ever had the man cave made fitted a FCU with single cable to CU so it is already there.
So if I want to add a greater than 13 amp supply to a ring final I could go around with the loop impedance tester and test the line - neutral impedance or prospective fault current on selected sockets and work out where the approximate centre is and extend from that point, but much depends on location of out building, if the CU is at north corner of house and outbuilding at south corner it may be getting cable to CU is hard and the south corner sockets are around mid way, so it may be worth stretching the rules, Appendix 15 says "The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:" and gives a set of points to be considered, it does not say you must follow the suggestions, it is part of the appendix not main part of regulations, however when you take the exam for the regulations the appendix is included.
Many places the regulations say must be taken into consideration, rather than saying must be followed, so if a manufacturer says this boiler must be supplied from a type A 30 mA RCD the regulations do not say you must do as manufacturer said, it says you must take what he says into consideration, however should it all go wrong, your left trying to explain why you did not do as recommended.
Best laid plans can go wrong, ring final at 30 meters, and you select centre for a spur to outbuilding, then there is an extension built and so the ring is extended to 70 meters, and now that outbuilding supply is only ¼ way around, so overloading of cable can take place. So in general it is better to follow the instructions and regulations.
