How did the bus drive from England to mainland Europe? I'm not sure I'd class the journey as "by bus" if it also includes ferries

I'm sure I'd class the journey by bus because the reference to the necessary and proportionally tiny boat or eurotunnel crossing would be seen as obvious, unnecessary and annoying.

- did you come by plane?

- not at all! i went by foot, took a bus, went by foot, took a train, went by foot, took an elevator, went by foot, took the plane, went by foot, took a taxi, and then by foot

- al..right

Don't also forget the walk to get to the bus.

The ferries would be brief enough though. Probably less than 100 out of the total 10,000 miles.

Can you stay in the bus when it’s on the ferry?

For crossing the Channel, definitely not. A very special exception can be made with prior arrangement for those with disabilities that made entering & leaving their vehicle too cumbersome, but they aren't going to ride this kind of bus.

No, but you can when the bus is on the train in the tunnel.

This was in the 50s; the tunnel still wasn't far off science fiction at that point.