Never even heard of a plain tomato sandwich.

In the UK we tend to pair tomato with either ham or even more commonly, cheese (inevitably cheddar). The best being a cheese and tomato "toastie" wherein the cheese is melted, the tomatoes grilled and sliced and the bread toasted and buttered.

We do however have the "cucumber sandwich" - finely sliced cucumber, buttered bread removed of it's crusts and cut into little triangles, though it's more of an old fashioned thing and less common now than it once was.