In response to the original question - I believe The Amphibious Flying Club are operating between Enniskillen in NI (EGAB) and thier UK base is probably the closest option. I believe they may do self fly hire for proficient pilots.
I would whole heartedly recommend Aero Club Como in Italy if you wish to combine a holiday with a check out. The Italians dont have the over powering attitudes that prevail in UK seaplane hierarcy. More like the US in that respect. Flying is just flying.
Just a comment on Seaplane flying in general in these parts, as I have my own seaplane at my house here in IRL.
It's a very limited, specialist, labour intensive, and expensive hobby.
I find it highly weather dependent, try taxing downwind in a 25 mph wind.... never mind docking. Its maintenance intensive and working on the plane at remote locations without services is awkward. Landing sites can be dangerous at best - there is more rock in some irish lakes than water itself.... and its a pretty lonely existance it you like that flying club scene.
But hey I do over 140hrs seaplane flying a year so I guess I must like it.
Regards,
WF