John Edgar wrote:
In what way is it illegal to kill vermin by drowning them? One of the
best ways I would have thought. I believe it is illegal to drown
humans, but grey squirrels?

It isn't illegal, although it is illegal to drown them "with intent to
inflict unnecessary suffering". Also, it does not take very long at all.
The alternative is to run it from the trap into a sack (a cloth one),
push it into a corner with your foot, and belt it with something.