Essex, my site follows a similar traffic pattern. I have seen the same happen last year as well. It tends to happen around all major holidays for me. I simply expect that I will see a drop in traffic during December and go on with life.
Holidays are slow for me too, especially the end of December/beginning of January. Then after the first week of January, it usually gets progressively busier until mid summer. I am preparing for that by stocking up and adding new products.
I expect my sites to be dead from now until early January, with traffic way down. Once the deadline for online ordering expires, pretty much everyone will rush to the shops or the pubs, before doing the whole Christmas thing. Might as well join them.
I find that most years the holiday traffic drops off around Dec 19th, 20th, or 21st. It generally drops to a lower level than most of the rest year. Traffic picks up again around the end of the first week of Jan.