lets have a go then,
To work out a more realistic fiqure, focus not on the number of circuits installed (which will be affected by the need to minimise: inconvience, circuit length, ring final inbalences, etc) but instead look at the minimum number of circuits actually needed
Very roughly (as the nature doesn't warrent being very precise)
One lighting circuit ground floor
One lighting circuit upper floor
One ring final for counter top outlets and applicances
One ring final for the rest of the house, space heating, small appliances, etc
One cooker circuit (9kw cooker)
One shower circuit (9.5kw shower)
one Immersion heater (3kw)
Ignoring smoke detectors, intruder alarms, gas heating systems for now
Outbuildings can be assumed to be on a FCU on the rest of house RFC
so we have
6A * 0.66 = 4A
6A * 0.66 = 4A
32A * 1 = 32A
32A * 0.4 = 13A
10A + (40-10)*0.3 + 5 = 25A
45A * 1 = 45A
13A * 1 = 13A
Total it up and we get 136A which is on the high side!
So consider whats likely to be happening?
Not kitchen full of applicances going, cooker on full, most of the lights on, shower on, vacuum cleaner going, space heater on in the dining room, and water heater on for sure!
So, if we start again and assume the shower is running, so thats 45A, ok, fair few applianes in the kitchen, say 20A worth, couple of elements on on the cooker, 15A, 8A worth elsewhere, 6A of lighting, 6A of applicances elsewhere and water heater is off
and we get 45+20+15+8+6+6 = 100A and looking at the discription above, that sounds very busy and is unlikely to be sustained for long
A 60A fuse will take 100A long enough for a bloke to shower anyway
Install 25mm tails, don't worry about the fuse, in the unlikely case that it does fail, the DNO will probably stick a bigger one in!