You're right. Parliament represents only the entire electorate. And only really those opinions that are shared by enough people to be worth an entire seat. Still, it's far better representation than you get in a district system, where only local majorities of the electorate in each district are represented.

So then in a country with a weak executive, who collectively represents the interests of non-residents, tourists, diplomats, etc.?

Because each group certainly has some real interests in common.