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:
(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.)
If we followed that advice we would have dedicated circuits for washing machine, tumble drier, dish washer, oven, and immersion heaters, in real life we tend only to put the immersion heater on a dedicated circuit, what is the main consideration is if there is an overload, will the protection device trip before the cable is damaged? When we plan a ring final we assume 20 amp centre load, and 12 amp even distributed, this will allow 106 meters of 2.5 mm² cable to be used, and be within volt drop, and unlikely to exceed the 1.36Ω limit to ensure the magnetic part of the trip will operate on a short circuit.
In the main electricians have done it so many times, they instinctively know when they as sailing close to the wind, so only bother working it out, when they know it will be a close thing. The calculations are not easy. The formula uses square roots etc, so I use a java script calculator also means less likely to make an error.
But for the DIY man the main hurdle is cost of the instruments, cheap (£50) plug in testers start at 1.9Ω but your looking with a ring final for 1.36Ω so you need an expensive loop impedance tester likely £200 for cheapest, you also need a RCD tester, can't really read 40 mS with a stop watch, and a insulation tester, so looking at around £600 worth of test gear, however if the DIY guy uses fused spurs then the £50 plug in tested will find most faults.
The installation certificate can have three signatures, one for design, one for installation, and one for inspection and testing, but in the main scheme providers don't use those forms, they only have one signature. However you are asking for some one who has never seen the site, to do the design for you.
All we can say is keep high loads near centre of the ring final, my house the kitchen is a good distance from the consumer unit, so all will be around the centre, so no problem, if the CU was in the kitchen, then more consideration is required.
Radials sound good, but volt drop limits a 20 amp radial to around 30 meters, where a 32 amp ring final can do 106 meters, so it needs three radials to replace one ring final, when we used MCB's and a common RCD it was not so bad, but with RCBO's the cost goes up as we increase the number of circuits, the ring final was introduced near end of second world war to reduce the amount of copper needed for the rebuild, little has changed, rings are still cheaper to install than radials both in cost and materials, I still remember my grand dads house with a fuse box with a row of rewire able fuses, one for each socket, and lamp circuit. And my dads house with 13A sockets instead of the 5 and 15 amp, had just 3 fuses, lights (5 amp) sockets (30 amp) immersion heater (15 amp) so much more compact, solid fuel cooker.
This house 14 RCBO's, 3 ring finals, 2 cookers, 2 showers, etc. Using radials I would double that.
