Thank you everyone for your replies. They are VERY helpful and I'd appreciate more, or as much information as other readers are prepared or able to provide !
The SMALL civil UAV in question would most likely be registered in the UK, though could operate anywhere in the World at low altitude (FL 0 - 120 as an example, in reality much lower), based primarily from the vessel (~80kt cruise speed only)
A query I have relates to Global State airspace boundaries. Is there a map showing the Global airspace boundaries of signatory states - or, rephrased, how might one know which State's airspace you were in whilst at sea, and hence which ATC Freqs. and Personages to apply to for an Airspace reservation etc. ?
It is expected that the Airworthiness aspect of the UAV will be performed in line with the CAA's CAP 722and its revisions if this required (for size and other reasons not discussed here).
(For information to guide responses following Spitoon's point, I had professionally been involved in Military Test Flying on the Engineering side for a few years and accumulated a fair degree of Airworthiness and general Aviation knowledge over that period, though not comparable to that of ATC or Military/Civil Aircrew, for example.)
