We travelled more than 2 hrs before in June. Our crossing was booked around 1pm ish and we turned up around 9.30am and got onto the next one (9.50?) free. Sounds like we were lucky though according to the other posts
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Eurotunnel are a funny lot. It has been known to arrive a day earlier than booked and get straight on, not charge but lately it is highly likely you will be turned away, or charged an extortionate amount of money to cross. Especially with high vehicles which means you will have even less flexibility than a regular car as there are less spaces. I would stay at 24/7 at Folkestone if it is possible & you need to then arrive pretty much at your booked time travel, this may well be the most stress free option. 24/7 is literally on the other side of the M20 and you can get breakfast and coffee before you leave for the tunnel. Enjoy whatever you choose to do!
We haven't been on the tunnel for a while, but they were always good to us. We travel a long way from Scotland to get there, and my wife is always worried about hold-ups and missing our crossing which means we can be there 4 hours early. We've always been offered the option to go right on, though they generally asked if we had travelled far.
One year coming home from France we decided on the spur of the moment to go to Disney for the day instead. I phoned them up and asked if we could delay about 14 hours. They never even took my name, just said swapping a daytime crossing for an overnight one was no problem. Sure enough we turned up around 3 in the morning and got straight on. It sounds as though they have tightened up a bit now though.