Except that at 10' asl, the pilot could not possibly see the submarine and therefore did not intend to overfly it.
Maybe, but rule 5 (3) (b) says:
(b) The 500 feet rule
Except with the written permission of the CAA, an aircraft shall not be flown closer than 500 feet to any person, vessel, vehicle or structure.
There's nothing about intention here. So you are in breach of the law if you are within 500 feet of a submarine or a caver, whether you know they're there or not. And saying you're on a 0.0000000000000001 degree approach to land doesn't excuse you. The exception for landing is 6 (a) (ii):
(ii) Any aircraft shall be exempt from the 500 feet rule when landing and taking-off in accordance with normal aviation practice or air-taxiing.
Note "normal aviation practice" and not "playing games with the law".
Interesting these mind games, what?