Take a spare set of bulbs and don’t forget the GB sticker (or is it Belgian sticker in your case?)
Saying this cause I got pulled about 3 years ago by cops just outside calais and they fined me 50 quid on the spot and made me buy a sticker and bulbs. I’d only been off the ferry for 20 minutes. No problem with the bike on the ferry though it was well strapped down. I might do the tunnel this year.