In the main cost of 2 extra RCBO's is neither here or there so it depends on how easy it is to thread the cables.
The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable this can generally be achieved by:-
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)
A ring final to be within the volt drop is limited to 106 meters of 2.5 mm² cable, rather an odd way to work it out, taken as being 20 amp centre load and 12 amp even spread.
Using 20 amp at end, a 20 amp radial can have 32 meters of 2.5 mm² cable. Could use a 16 amp overload which lifts it to 42 meters, but this is the problem with a radial the volt drop means far less cable can be used. Could of course wire it in 4 mm² so at 20 amp 55 meters can be used.
However 13 amp sockets and socket boxes are designed for 2.5 mm² cable, using heaver cable presents some physical problems getting the cable into back boxes and sockets, MK will take 3 x 4 mm² or 2 x 6 mm² but not all makes will, they only need to allow 3 x 2.5 mm² or 2 x 4 mm² so using thicker cable can present problems.
End of day the 13 amp socket was designed to use 7/0.014 cable which is between 2.5 mm² and 4 mm² using a ring final, the old 15 amp socket required so much cable and so many fuses seems prudent to stick with the ring final.
