i REALLY wouldnt use USPS for shipments to the UK. Why not just spend an extra $5 or so and get FedEX 1-3 day shipping?
Just so u know, your USPS tracking number will work on the Parcelforce webpage (they deliver USPS parcels in the UK).
I cant recommend FedEx enough tho, it's a little more but the tracking is far far better (you can even track the flight its on), and deliveries get to my door in 48 hours after placing the order.
I actually ordered at the same time from NP and UK supplier CHUKS. The NP order arrived first!!