Reverting to the original question, prawn sandwiches are very popular here in the UK. They are usually whole prawns with mayo and lettuce and presented between two slices of bread. There are prawns and prawns of course. We're not talking king prawns. Open sandwiches were not really known in the UK until probably the late 1950s when they were introduced from Scandinavia, but I would say they haven't really caught on here.

Can't stand banana sandwiches. Can't stand peanut butter sandwiches. A mix of the two must be frightful.