I'm away to Santander in September (2020 booking!!!), 24 hour (can't do the 36 hour - would drive me crazy!!). Bike will be tied down, but I always make sure I put my gloves under the straps, even with a cushion. Some crossings prefer the bike to be on the side stand, and then force the bike down on that side. Always been worried about that but no ill effects so far...
Back a few weeks later Calais - Dover, Irish Ferries (£34). Will NOT entertain P&O.
The advantage of a ferry over tunnel - apart from price - is that you get chance to relax, grab a coffee and a sarnie, and rest the bum! Much better than sitting on the floor of a dirty carriage hoping your bike doesn't go over if they brake suddenly.
Just my tuppence worth...