The "We've gone on holiday by mistake!" is one of the subtler gags of Bruce's finely crafted script.
Everyone knows that you can't go on holiday by mistake*. 'Going on holiday' is something you set out to do. The gag being: they chose to go on holiday, and when they got there (well, Withnail in particular) they realised it was a mistake. It's a bastardisation: hence the killer line.
Bruce says the line usually only gets laughs from English Language students. Most people miss it. I missed it the first few occassions!
*Though I would not be surprised if Withwine begs to differ.
"Will we never be set free?"