Ça alors! Why was that? I've criss-crossed the old Manche any number of times, doing Roly's Famous Christmas Tree Act, encumbered by everything from hurdy-gurdy to rauschpfeife and never been expected to declare a thing, other than, what does a middle-aged man think he's doing, travelling by himself like that? (Can't be up to any good. Must be a crazed, drug-dealing Muslim fundamentalist paedophile suicide bomber with incurable bed-wetting...) I respect Her Majesty's Customs and Excise and Immigration services. They have a job to do. I'd sooner they didn't do it all over me...
I urge all you non-Brits: never, never, say "I went to a festival" - even if it's a Bach organ festival - in the earshot of the U.K. border authorities. The word "festival" has the same effect as a pig gate-crashing a Bar-Mitzvah. And of course the x-ray image of a concertina never fails to create an impression.
Best wishes, and sorry to digress.
Big Roly