The ferry companies will typically put you on the next boat if you turn up a few hours early and I would be very surprised if they made you wait - the last thing they want is someone hanging around for hours on end. However, they do offer cheaper deals out of peak hours and if you go for the cheapest deal then try to travel at a peak hour they may not let you - perhaps you asked for cheapest possible crossing on the web site and that's why it offered you those unpopular times only? Even in February when Calais had 3 out of 5 ferry berths closed due to safety check and there were miles (literally) of lorries queuing, I was put on the next boat out in my car when I arrived a couple of hours earlier than booked from Munich.
My comment about "charging back" your card: There are some very good deals such as "24 hours for 24 euro" return tickets that they sometimes offer. If you buy one of these but only use the outbound ticket, they'll make a further charge on your credit card when you don't turn up for the return leg, so that you end up paying a standard single which is more expensive.
The 18 hours I quoted was my door to door timing - so it includes the Ferry and UK legs. Don't think I was very clear on that. But I would still strongly advise on a first such drive that you plan to stop overnight on the way - perhaps near the Belgium / Germany border - rather than taking any chances of falling asleep at the wheel - we want you back
![Grin ;D](https://www.talk.uk-yankee.com/Smileys/classic/grin.gif)
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Although it's your first such long drive in Europe, thousands of people do it and it'll turn out to be much easier that you imagine.
Enjoy the trip!