I'm not keen on flying, but it's fast. this page has several alternatives:
I suspect that in mid-summer none of these options will be especially cheap and all of them will be crowded. Anyone got any experience of any of them? Anyone organizing a group rate deal by any route (including air if necessary)?
later - It just occurred to me that by then it's possible that we'll be in the middle of Brexit - crossing several borders with countries that are annoyed with the UK would not be fun... From that point of view a flight might be the better option.