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It's a superb experience. About 8 or 9 miles depending on where the safe place to cross the Kent channel is. Once out at the channel quite a few miles out to sea it very eerie and you feel very vulnerable. The timing has to be spot on to within about 10min or you'll not make it to the other side before the tide comes in! Thankfully Cedric Robinson, the Queen's Guide to the Sands, knows his stuff!
I did that walk many years ago. I think I was around 13 years old. My sister and I went with a group of adults and a guide. Funny thing, I can´t remember at all how we got back !