There are quite a few places that offer a seaplane rating to add to the PPL. Caledonian Seaplanes (I think) is one in the UK, Google should find it.
Otherwise it's off to the US or Canada where it's cheaper but with the associated paperwork and red tape. Have a look in the back of Pilot or Flyer for the schools, I can reccomend Jack Brown's.
Good luck,
GA