Quite separate to any approvals of an aviation nature, there are significantly less planning restrictions on moveable structures, and as we are talking of a ship, does it need planning permission to moor? Just a thought....
LCY has had helicopter visitors on some of its summer open days (DennisK did a display once or twice I recall?), and I believe the police or air ambulance are exempt from the planning restriction. There may have been a declared emergency nearby which necessitated a landing... cannot remember the details of that one, but think it was on this rumour network some years back.