Lets not get ahead of things here, first of all a sub CU fed by a FCU is not a common set-up, especially off a RFC. In fact I don't recall ever seeing it other than when the feed has changed for some reason or the building was prewired such as a Portacabin. To put it bluntly it is generally a complete waste of effort and money as no additional protection is offered [with caveats].
I predict that EFL didn't give a definitative value as there has been nowhere near enough information provided by you to do so, I simply offered the maximum permissable.
The RFC [originally called ringmain] and BS1362 & BS1363 were created for each other and the original maximum load was 13A but now the next standard sized MCB has become acceptable.
What you really have to learn is the amount of information that is now required in electrical design, in the old days it was easy: 15A socket, 15A fusewire & 7/0.029 etc. It aint like that now.
Your OP is very much akin to 'What flavour are red sweets?' So when you don't get the answer you want to hear don't complain, when these very experienced people ask for more information it means you haven't given enough. I will reiterate that and say I've witnessed a number of your questions where you have not given the needed information.
So please, for all of our sanity, stop complaining. If this is not plain enough for you the next answer will be.