If the app in question only supports US listing currently, then yes, only US listings will be shown.
However, if the app supports UK listings alongside US listings, it will show UK listings.
Buying the application on a US account and syncing to a UK iPhone has no affect (in any way) on the standard,
built in, region support of the application.
I tested the reminder feature today. Wow, works great. I took some pictures of stuff around the house and got replies within an hour. Now time to go to Best Buy .