Because the author of the Moomin books, the late Mrs. Tove Jansson, is a Finn, you can only find Moomin things from Finland. The Moomin theme park you are talking about is in a city called Naantali here in Finland, it is easy to go there from Turku for example (by bus). The park welcomes both children and adults, so it is not just for kids!!! The best bet to travel is to take a flight from London to Helsinki first, and then either take a train or a bus to Turku or a bus directly to Naantali.