A 7700 squawk is going to be visible over a huge distance.
London Control for example see a long way into the land area of N France i.e. way across the Channel.
This accident doesn't make sense, but then a lot of things that some pilots do don't make sense. For all we know he might have been flying at 1000ft over the water. That sort of thing will be in the AAIB report, but by the time that comes out most people will have forgotten about it.
The police comments are very interesting. They must have a reason for saying that.