Generally, what I have found with USPS to the UK, is that the tracking is fairly accurate until it leaves the US, and then nothing much after. My last one was a WASD first class, and I was checking every day for an indication it was in Britain, when it just arrived at the door about 4 days after it was still showing as departed Sort Facility.
I am in the same position with 2 other parcels now, one showing Depart USPS Sort Facility - July 31, 2013. I expect it to arrive at the door sometime soon, even if the tracking is off. The other one has about 3 different tracking places, last departed 2 days ago.