The word for floors in my native language is the same as the word for “upstairs”. And it’s the same in a lot of European languages (French étage). And the ground floor has its own separate word. Using French as an example again, the word is parterre, literally meaning “on the ground”. So numbering in Europe goes ground floor, first floor, and so on. Now in English, the word being the same, it can sound confusing, but I assume the British just adopted the same system as the rest of Europe for consistency (although they’re not usually known for doing that).