I have a 60 amp supply and the fuse has never blown. I would think most houses are the same. In real terms we will not worry about the supply fuse until it blows and in most cases the DNO will upgrade a fuse if it blows twice.
If we follow the diversity given in the books, most 10 and 12 way consumer units will fail to pass. I have with very large houses used a three phase switched fuse box with three fuses at 100 amp each and linked all three phases on input so it could be supplied with a 160 amp DNO fuse without fear of over loading the three consumer units or could be converted to three phase. Since the consumer unit is ratted at 100A unless you use a method like this a 100A DNO fuse is limit.
On return, once the house had been occupied, we found there was no problem with 100A fuse the DNO had fitted.
As to counting up fuse/MCB sizes at the consumer unit this will give errors. Since one is only allowed one spur per connection into ring main in theory one should not have four or six 20A switches feeding dedicated sockets for Washer, Tumble dryer, Dish washer, Fridge, etc from the ring main. (Although it is common) You should have individual feeds to each switch so you could likely have 4 x 2.5mm cables all coming from 16A MCB’s feeding the same grid switch assembly, as many houses have fed from a kitchen ring main. Clearly, it uses no more power than houses where it is feed from ring main, so there is no need to consider the 4 x 16A MCB’s as having a 64A load. In real terms unlikely to exceed 32A. The same applies to immersion heaters which if left switched on will likely not run long enough to really effect the total load.
So in real terms all the sums shown in the books as to how to calculate diversity don’t really help.
The only thing that will likely cause problems is multi-electric showers all used together as family prepares to go out. And a water storage tank even with standard immersion heater will alleviate that. Hardly worth a bigger DNO fuse just for the showers!
