Inbound numbers have CallerIDs. This is passed to you by the Carrier. If you have a different carrier for the UK phone number, they may be passing the callerid to you in a different manner than your US carrier.
Posting a ClI example of each of the two types of calls (US with CID and UK without CID) may present you with your solution. Failing anything obvious on the Asterisk CLI, you may need to turn on SIP debugging and get a bit more detail regarding the initial handshake during the "invite" which may show you the technical differences more clearly. Sometimes it's necessary to put a special line of code into 'trunkinbound' to capture the CID from 'special' carrier who write their own rules.