This thread has been rather amusing
As far as I'm aware, most international airmail leaves the UK the day after posting. The reason your letters are taking longer than usual to arrive is due to a problem in the US that is effecting inbound mail from all countries, not just the UK!
Due to a failed parcel bomb back in November, The United States Department of Homeland Security is continuing to require increased security measures for items carried on passenger airlines. As a result, some 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.
So it's highly likely this is the reason your letters have been delayed - nothing to do with Royal Mail.