There's one in Scotland (Caledonian Seaplanes). Most people go to BC or possibly Alaska.
Costs are, oh, upwards of twice the price of landplanes. Hours for a UK seaplane rating are not-very-many, plus a seamanship exam, details in LASORS.
Finding someone to let you fly solo in their seaplane round here will not be easy (solo is not required for the UK rating - it is however required for the Canadian rating, so if you succeed in getting the rating in Canada this includes five circuits solo).