If you are thinking of moving to a National Deep brood box ie 14" x 12" then you may have delivery problems. Thornes tell us they are completely out of, and working flat out to supply orders. They have been completely surprised by a big increase in demand for the larger brood box. It would seem most of the orders come from Hertfordshire! They hope to be fully stocked again by next month.
If you too are thinking of getting a larger brood box, then you might want to consider a Modified Commercial brood box instead. The MC is only a few mms different in size from National and you can use your old National supers. Just beware of top or bottom bee space. Foundation size is 16" x 10" giving a very useful brood area. Interestingly, the Commercial/National combination is very popular in Essex. It just goes to show the marketing effects of your local guru.