The problem isn't with whether in practice, the load ever gets close to the maximum the supply will support, it's with the calculated maximum load, allowing for diversity, as per BS7671.

I can't sign off an EIC for something like a charge point installation if I know that the sum of the loads, allowing for diversity, where applicable, exceeds the rating of the supply. A 60 A supply may well be able to support the actual demand, plus a charge point, but if the sum of the loads for each connected circuit exceeds 60 A then the installation is non-compliant.

The loads aren't hard to calculate, just means looking at the individual circuits and then doing the sums to work out the load for each. For example, each ring final circuit, on a 32 A MCB (or maybe a 30 A fuse) will be calculated as 100% of the total current up to 10 A, plus 50% of any current over 10 A, so for a 32 A MCB the calculated load will be 10 A + (22 A x 50%) = 21 A. For a lighting circuit the calculation is 66% of the total current, so for a normal 6 A MCB the lighting circuit load will be assumed to be 6 A x 66% = 3.96 A (rounded to 4 A). A cooker load will be calculated as being 10 A plus 30% of the current over 10 A, plus an extra 5 A if there is a socket on the cooker outlet, so for a fairly standard 40 A cooker circuit, with no socket at the switch, the load would be 10 A + (30 A x 30%) = 19 A.

The problem loads are heaters, like electric showers, or immersion heaters, where no diversity is allowable. A 9 kW shower will draw just over 39 A, and an immersion heater will draw just over 13 A, at the UK nominal 230 VAC supply. There's no diversity allowable for electric floor heating, etc, either. The same applies to a charge point, there is no diversity allowable, so the full current has to be used in the load calculation.

Here's an example calculation for a fairly typical house that has two 32 A power ring final circuits, two 6 A lighting circuits, plus a 40 A cooker outlet:

Each ring final is 21 A, so sub-total = 42 A

Each lighting circuit is 4 A, so sub-total = 8 A

Cooker circuit (no additional socket) = 19 A

Total = 69 A, so already over the maximum allowable for a 60 A incomer. The chances are that many houses on a 60 A supply may have either a restricted number of circuits, or have the maximum load on some circuits reduced, in order to stay within the supply limit.