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
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.
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.