Solving the problems
The EU FUSETRA (Future Seaplane Traffic) programme is interesting re law and seaplane bases and well worth googling, but I think the future could offer much more....
As far as I can tell, wings off or wings folded, a seaplane is a boat, not a seaplane. This would allow taxiing in harbours and estuaries, also enabling physical access to marinas and docks.
Composites are used for building high-spec yachts identically to light aircraft (diamond, Cirrus etc) which will deal with airframe corrosion when used by seaplanes.
New hull forms derived from racing multihull yachts have been developed for seaplanes, which will be i) lighter ii) less drag iii) far more seaworthy.
Used in amphibs (to enable airfield use for inland options), would these serve, even in England and Wales?