I’m unreliably informed that the absolute minimum amount of liquid to drown a human is 1 liter. That might require a special head-shaped bucket, but it seems plausible.

But out of curiosity, what sort of statistics are you seeing about UK people falling into canals? I know they have canals and people, but I thought the trope was shopping trolleys (USA: shopping carts) falling into canals. Is this a serious issue? Can we find comparable figures from the canal-strewn Netherlands for comparison?