Neutral and earth

Because they serve two completely different purposes. Neutral is a current carrying wire in normal use. Earth should not be current carrying, and is there as a path for current to pass under fault conditions. Neutral can be switched, earth should be a permanent connection. Neutral is classed as a "live" wire, earth is not etc.

That depends on the earthing system in use; with TN-C-S (PME) then yes they are combined at the main cutout where the service enters the building.

With TN-S, they are combined at a substation, not at the building.

With TT, there is no earth provided at all by the suppler, you will need your own arrangement, and it is not necessarily connected to neutral at all.

There is an earthing system TN-C (Where T = "Terre" aka Earth, N = Neutral, C = Combined), that does work as you describe - no separate earth, and neutral used as earth directly. However I don't think I have ever seen an installation like that in the UK, it is very uncommon.

For more detail, see:

You could switch off the isolator so all systems are dead and measure the resistance between the black and earth with a bonding meter.

You would need to disconnect the main EQ bonding to get a reliable reading. Otherwise you could find yourself measuring the loop impedance through someone else's TN-C-S earth indirectly.

One would need to measure it first to determine if resistance is in the correct range.