What does it say on the Royal Mail website? They may have more info.
You could also call USPS and they may be able to give you more information. Rarely, though. They pretty much have the same info as the public website, but once I had a shipment that had a vague description like yours and they said when I called that they were able to see it was in customs.
Also, Registered Mail may have small delays and be tracked differently, as it has to be signed by an employee at every stage and kept in a locked part of the building. I never use it; it's not worthwhile.