The insurance thing is a little more extensive than that.
Basically amphibs are subject to all the usual risks of GA PLUS those associated with water. Just a guess but judging from the incidence amongst our group, being amphibs probably doubles or worse the chances of an accident. Also incidents on water are usually considerably more expensive than regular landing incidents. Recovery, dry out, engine restoration or replacement and a higher proportion of write offs prejudice insurance companies against water birds.
I have no idea how the permit system works in the UK. Here in Canada we have Ultralights and 'Amateur Built.' I chose to build jemima under the Amateur built which is basically the same as the USA "Experimental." Essentially, if you can show the aircraft complies with regular good practice you can get the permit. A number of aircraft designs are considered acceptable based on experience and that includes the Searey.
Almost everything on the Searey is built by bolting together Al tubes and covering with cloth (Wings etc|) or a premade fibreglass shell (fuselage.) I'd say it was in the order of an IKEA or MFI kitchen.