You obviously don't know BA very well MOR. There are plenty of things they could sack the Captain even with a zero blood test. They'd throw every rule in the book at him until they could fnid something that stuck, then they'd fire him, then he'd have to go to an industrial tribunal, and even if he won BA are still not required to re-employ him and it would require industrial action by the pilots to force his reinstatement. Perhaps he simply decided it wasn't worth all the stress, especially as the last guy who went through the tribunal process dropped dead as a result of the stress.