you really need to give careful consideration when calculating maximum demand and diversity, as if you go too strictly on bs7671 guideline amounts then you will end up with huge a demand when in reality the total load will never reach anywhere near that amount, you also must consider the likely use for the installation too, there is not a formulae that will be accurate for every eventuality, for example an upstairs 32a ring circuit in a domestic property will likely never see close to 20A for any period of time let alone 32A!, but in a big kitchen it is more likely to be higher and for a longer duration, adding up mcb ratings will just end up with silly figures, you have to assess the installation and use your experience, things to look for in a domestic property are generally kitchen ring mains, electric showers, immersion tanks, electric cookers and electric heating, things like smoke detectors i wouldnt personally even consider.