Consider Canada. I recommend
Lake Country Airways: floatplane training, endorsements, float charters and rentals, it's an excellent flying environment - very relaxed, and no waiting. if you come in the next weeks, the fall colours are spectacular.
I don't know how an FAA or UK license would get endorsed with the float rating, but it won't matter a lot, as you would not likely be flying solo anyway - insurance.
If you need more information about flying in this area, you may PM me.
Pilot DAR