I've always found that for some reason when I send a parcel to the US it takes ages, whereas they make it to the UK in under a week. I've had simple packages take three weeks to arrive in major US cities... have never quite worked out why it takes so long but it just does...

And also I imagine that with the increased alert stuff they are possibly being even more cautious than usual with mail between UK and US and it may be taking longer than normal to process.