From the 15th June 2007 anyone travelling to or from a country outside the EU has to declare any sums of cash exceeding 10,000 or more (and equivalent in other currency).
Briecase openCash will only be seized if HMRC have reasonable grounds to suspect it is the proceeds of, or is intended for use in unlawful conduct.
Now someone close to me recently bought a house abroad, and the rather dodgy builder he ended up with insisted on being paid in cash. Result briefcase stuffed full of cash onto aeroplane. Now would HMRC have reasonable grounds to assume that this builder was not going to declare the income, making the transaction illegal and thereby seize the cash, even though tax and NI had already been paid on every penny of the money earned to fill this briefcase?
As it happens, in this case there would have been absolutely nothing to worry about anyway, as the house was in Italy and therefore in the EU!
Actually I don't think there could be anything illegal about paying in cash, the builder is later behaving illegaly in his failure to declare the income.
However, if HMRC were to phone the builder from the airport, just to check out the story, what do you think he is likely to tell them?