Not sure if the flight would have been talking to London Information but probably so, especially planning to do an over water flight. I am assuming he had reported his planned route as via Caenarvon bay but I wonder if he actually reported coasting out?
It is possible the accident happened prior to that and possible CFIT in Snowdonia hence the present ground search now.
I guess the radar tapes would have been pulled and checked though this only of us if within radar contact so this may give an idea of last area of contact.
As a voice of Scottish Information I speak to these guys on a daily basis so we all take it to heart when something like this happens.