The British isles consists of both Ireland and the United Kingdom.

The odds you picked the correct door at the start is 1/1000, that means there’s a 999/1000 chance it’s in one of the other 999 doors. If the man opens 998 doors and leaves one left then that door has 999/1000 chance of having the prize.

I swear my first memory is falling down the stairs but neither of my parents can remember it ever happening