As The Indian Girl pointed out, we don't use the term "flatmate" because over here we don't call apartments or living spaces flats, as they do in some countries.
"Roommate" seems to make sense because even if you don't share a personal room, you still share common rooms in the house -- the living room, the kitchen, the laundry room, etc.
Or, as The Indian Girl also pointed out, we also use the term "housemates."