A few years ago now, I was doing a survey for Marine life, on the Newhaven Dieppe ferry.
It was September, and an Osprey was disturbed from the ship, below the bridge, about an hour into the crossing.
It's unlikely the bird would have landed on the ship in port, or the first few minutes of the crossing, so our theory was that it probably got a fair way across the channel before landing on the ship during the ship's previous north bound journey!
Then sadly was disturbed at the start of its southbound ride, having to continue it's journey from further North than it had previously made it to.
So, in this case, the hitchike would have given him a rest, but in return for a longer crossing.
However, the northbound ferry is overnight (unless it was delayed!!), so would have implied overnight migration over the channel by the Osprey.
So, Maybe it did start it's journey from England, then landed on the ship about 10 miles over the channel, then left soon afterwards.
We will never know!