As a PPL I've never really understood the customs airports things. From what I derived from my air law book was that to fly to another state i.e to France from the UK, once must leave via and arrive at a customs airport.
However, I flew to France (Le Touquet) with an instructor and none of this was necessary. And out all all the things I asked him, I forgot to ask why we didn't need to leave via customs airport. I don't know if Le Touquet is a custom airport, but all they asked for was a landing fee and aircraft reg.
Can somebody clarify?