I'm rather fat, and on some Airbus' (shouldn't that be 'Airbi' for the plural?), I need a seat belt extender, and others I don't. So I actually measured one seat belt one day. At maximum extension, it was 36 inches long.........(this wasn't on BA, but a US loco)
As the size of people generally has increased (bigger bone structures than 100 years ago, ignoring fat) you'd thonk the airlines would have figured this out. What is needed now is a regulation on minimum seat width and a minimum length seatbelt.