Ther's a seaplane operator in NI -
Lakeland Seaplane Tours - Enniskillen
...and there's landing rights available in Kent -
Seaplane landings on the Medway restricted - Yachting Monthly
It appears that someone operates a seaplane on Lough Neagh (the largest body of fresh water in the UK) -
https://500px.com/photo/40727970/sea...-by-tam-mullen - sadly there's no more info than that.