So I also had this issue. Im not sure if it has fixed the issue yet as only logged on tonight to sort it.
But Google Merchant Centre Dashboard
Then Products
Then Feeds
Click your feed name that is sent by Squarespace it doesnt look like or isnt labelled as an edit field. but if you click the feed name it takes you to a details page
it then shows target countries, Click the pencil, Change it from US to UK. Click save Your Target destinations is now UK.
Ill wait for the reindexing but that does look like it will work.