Hi Katarina,
You need to set up two GMB locations. One location has to be in the USA and another one has to be in the UK. Also, you have to have a physical location in both countries in order to receive the postcards from Google and verify your listings.
You can add www.example.com/us/ version of the site to the "website field "in the GMB, and you add a regular version of the site to another GMB( UK Version).
Ross