Seaplanes and the UK
The underlying problem, as I understand it, is the small but significant difference in US and UK laws.
What is not specifically prohibited in the US, is allowed.
What is not specifically allowed in the UK, is prohibited.
So, I can barge around the US in my 1967 PA18 on Wipline floats and have lots of places to land and play. An owner of a lake can prohibit me, but in most cases they haven't considered the need, so no prohibition exists. Pretty much all Corps of Engineers lakes are allowed, as property owned by the public.
Last Fall I landed on a lake and a conservation officer came out and asked if the lake was permitted for airplane operations. I told him there was no such thing. Was he aware of it being prohibited? At this moment in time, I was a boat. He thought for a while, and asked how fast I would be going at take off, and I told him about 40MPH. He agreed that was within the speed limits for his waters. Then he told me there was a "launch fee", to which I pointed out the obvious that I hadn't "launched". After some head-shaking and joke sharing, he gave up and left us enjoying the sun on the beach.
I understand there is a fellow up in Scotland that gives lessons in a PA18, although he may have taken a break for a while. Otherwise, I can't see any investment in a floatplane making sense in the UK. Waste of money, and they can be costly to acquire and operate.
Cheers,