Final, you're looking at Rule 6 (b)
See
http://www.caa.co.uk/docs/1676/Rule%...il%202005a.pdf
In your scenario, assuming there's nothing pertinent that you're not telling us, I cannot see why any of the exemptions in Rule 6 would apply.
Except that at 10' asl, the pilot could not possibly see the submarine and therefore did not intend to overfly it.
So, if a pilot overflies a cave, with a couple of potholers in it at less than 500', is s/he in breach of rule 5?