I'll soon be continuing my world hitchhiking trip from Germany to Scotland.
When I need to use ferries I try to find one that charges per car because I can try to hitch a ride with somebody taking their car on the ferry.
If there is no such ferry then I'd also like to know and I'll put aside some money for the ticket.
A quick check on the most basic ferry of all, Dover-Calais with P&O, reveals that the cost of a car is the same no matter how many people are in in it. Simply check the price on this site and vary the number of people. I suspect that most other routes are the same.
However you do have to book the number of people, so your plan to hitch a ride is unlikely to work. And security considerations would probably prevent it happening anyway.
The Eurotunnel might be your best bet as they do charge per car (and up to 9 passengers.)
It is also the quickest crossing.
That said, security is quite heavy on channel crossings, and the penalties for people smuggling high so I suspect that it will not be easy to find someone willing to offer a lift.
You can also use the train ferry which is subsidised and not super expensive to the Netherlands and Ireland:
It’s probably best not to try and circumvent the ferry rules when you’re travelling across an international border, they could deport you which would be very bad.
