As someone who will die on the hill that USC/Imperial is worse than (or the same as) metric in every single way:

Yeah, the British are idiots, and we Australians also use their confusing system too. I hate it.

The ground level is the first level you walk into, this should be 1.

Expressed another way:

— 2

Level 2: between floor (the actual floor) (1,2)

— 1

Level 1: (0,1)

— 0, The ground

Level B1: (-1,0)

— -1

Etc

In the international system (the one Americans use) you are concerned where your head is.

The British system wants to know where your feet are.

The American (and many other countries) system makes way more sense.

The ground floor is the first floor.