When to leave?
Firstly you need to try and leave London as late as possible. The later you leave London the more time you'll have on the French side to do it within 24 hours. The ferry from Newhaven leaves at 11pm so you need to be there at 10pm and it's about 65 miles away. I'd give yourself 5 hours minimum, or 6 hours to be safe and have a pub stop on the way. So if you leave London at around 4pm you only need to reach Paris 5pm French time.
You don't need much kit at all. View my KIT LIST here. Keep it simple. Don't take any casual clothing.
Time off the bike
This will most certainly be the make or break of making it in 24 hours and often something we all overlook. I remember once in a race I was pushing hard with another rider. I had to stop for a pee and it took me 45 minutes to catch him again. Remember you lose at least two hours with trying to get out of London and into Paris. Better to keep a good steady pace without stopping rather than sprint and stop a lot. It's the pace that kills, not the distance.
In order to do the 190 mile (300km) ride in 24 hours you should really have done two 100+ mile rides in a row. If you can do that then your are mostly there. Although 24 hours you only really have 18 hours because of the ferry.
I always struggle to sleep on ferries, especially when I'm in the middle of an adventure. My mind is excited and busy taking it all in. I now know this so spend 2 or 3 days sleep banking. There are various theories on this but I've found that if I have 3 nights of 12+ hours sleep I can then have 2 hours sleep on the 4th night and won't feel sleep deprived. I only managed half an hour sleep on the ferry and then another half an hour in a Tabac. This was enough in the end.
Food and Drink
You must drink at least 500ml per hour which means drinking well over 6 litres. Important to hydrate. You can often forget to drink on these long rides because you're normally used to drinking only 2 bottles per ride.
Also make sure you eat A LOT. You need to take in around 5000kcal. Bananas, chorizo, flapjacks are all good. I also took some gels which I saved for the final leg into Paris when I just wanted to get there.
This is also very important. It's always a bit hit and miss when planning a route. BUT you must plan one and if you can have it on GPS the better. This saves a lot of time. My one tip is you MUST do Ave. Verte which starts at Dieppe. It's incredible and consists of 30 miles of paved over train track. The added benefit is you'll be doing this in the dark after not much sleep so it's nice not to have to deal with traffic.
I've included the GPX files for our route below. All in all they were pretty good. I'd give them a 8/10. Most of the route was pretty amazing but we did go down some pretty busy duel carriageways into Paris where you needed to be quite confident. Also there were three occasions where it took us down dirt tracks. These were easily avoidable as we diverted around them without too much hassle.
Distance: 190 miles (300km)
Total time taken: 23hrs 15mins
Actual cycle time: 13 hours
Sleep: 1 hour (It's possible to have 3 if you can sleep on ferries)
Liquid intake: 6-8 litres
Calories burned: 5000kcal
Average cycle pace: Around 14.5mph