UK detains journalists based on Anti-Terror Law

As David Miranda isn't a journalist then the OPs headline isn't accurate.

The more interesting question is why did he decide to fly though London with the leaked documents, why not choose a route through Spain or further south?

Quoting for emphasis:

"I'm convinced they've hurt their own country far more than me with this ruling, as it emphasises what the world already knows: the UK has contempt for basic press freedoms." — David Miranda