I ordered a Captains Choice lather bowl a couple of months ago from the US and that came via USPS, and then via Royal Mail, once it had cleared UK customs.
Mine came from the US via a few stops in the US, and then over the Atlantic to the UK. Once it hit Heathrow (customs) it was handed to Royal Mail. Think in total my parcel took 10 days to arrive.
By the sounds of your parcel, it has mistakenly got sent back out on the outward mail from customs.
From memory, the Royal Mail had my parcel at my local delivery office for around 1 day, and the next day I got a card put through my door, saying that there is customs fees to pay, and how much, and that it was available to collect.