From the Royal Mail website
"USA
Start date: 18th November
End date: Ongoing
Increased security measures delays some mail to USA
Since November, the United States Department of Homeland Security has increased security measures for items carried on passenger airlines. As a result, mail entering the US from around the world, and including the UK, that would normally be sent via passenger aircraft, must now travel by other means, including ships and cargo planes.
The vast majority of items, including letters and Christmas cards, posted prior to the last recommended posting date of 10 December have been and will continue to arrive in the US in time for Christmas. However, these ongoing security issues, combined with the impact of recent severe weather on the movement of mail throughout the UK, are causing delays to some items bound for the US. These delays mainly affect heavier packets, but other items including Destination Sort bags posted under contract by business customers are also affected. We’re really sorry for these unavoidable delays and are doing everything we can to keep mail moving out of and into the UK.
Since the middle of November and throughout the recent period of weather related service disruption in parts of the UK, we have implemented a number of contingency plans to ensure as much of our customers’ mail reaches the US as quickly as possible. In finding alternative, available routes and transportation to the US, it’s possible that the delivery pattern of some items may have been interrupted and not all mail will arrive in the sequence in which it was posted. Handling significantly increased volumes of mail at this festive time along with the complexity of reacting at pace to contingency arrangements, means we’re unable to track individual items for customers or identify exactly when in the current arrival patterns deliveries are likely to ta