The method of calculation of diversity as found in the OSG is very conservative. In reality the maximum demand is far lower than this. If you really want to know, put a recording clamp meter on the tails. I would guess that most installations peak at around 30-40 amps.
The only exception would be on Christmas Day when the turkey is in the oven, all the cooker rings are on, the washing machine is going, the kids are playing with their new electronic toys dad says to mum "put the kettle on while I have a shower". You may then pop an old fatigued company fuse. There is a massive margin of safety in tabulated cable ratings. So if the installation conforms to BS7671 and the terminations on the consumer unit are tight there should be no problems. The peaks will be only of short duration.
A 100A fuse will carry 200A long enough to boil a kettle, and 150A long enough for a good shower, so it won't really pop. (check the curves) And that 'Christmas Day' scenario isn't that common either, or we'd be losing substations like a string of fairy lights - remember that typically the transformers are sized on a working load of only couple of kilowatts per house -a logic which they extend to some extent to the sizing of the street conductors too, (only for substations or cables feeding 20+ properties though, to allow load peaks to smooth out)
Remember also on the cooker, the rings are not on all the time - their thermostatic controllers see to that, so once the water in the pan is boiling, whoever turns down the temp, they just ping on and off, with perhaps 10 to 20% duty cycle.
Don't panic - many houses calculate up at >200A and run quite happily on a 100A, or even a 60A service fuse. And if not, and one day it does pop, then the supply company will come out and fit another one, or the next size up (80A, 100A 125A...)
You can estimate it, which is what experienced electricians normally do. or: -
You can work it out using the OSG for reference regarding to diversity etc; or: -
You can just put the main fuse size down, such as 60 Amp, 100 Amp etc; which many do.
Jaymack