I emailed CLT customer services and received the following response:
I checked with US Airways’ Senior Manager for International Flights here in Charlotte and was told that the minimum time to clear Customs is 45 minutes. Typically, the official connecting time off a transatlantic flight is 70 minutes. Additionally, the order you get off the plane could be a factor in the time as well. Meaning, if you get off the plane towards the end, you have that many people already in front of you for clearance.
I don't really want to risk such a tight connection so I guess I'll ring bmi tomorrow and allow them to change me the £20pp change fee.
Many, many thanks for all of the input^