I know this was discussed last year, but it's bound to come up again.
P+O are stricter than Norfolk Line, and the Chunnel are even worse, when it comes to carrying things.
LPG:
Advise the ferry company WHEN YOU BOOK if you're driving an LPG powered vehicle. Norfolk Line just want to know beforehand, and allow vehicles with fitted LPG tanks, or one removable refillable LPG tank.
Fuel:
No fuel in cans (including vapours) on Chunnel, P+O aren't keen, Norfolk Line don't care. Jerry cans should be inside the vehicle, or on the roof - not somewhere where they could be easily ruptured in a crash (so not on the back door). No point taking petrol or diesel with you - it's cheaper there.
Cooking gas:
Norfolk line should be told WHEN YOU BOOK if you are taking a refillable tank. One refillable tank is allowed, and must be stowed in a safe manner, closed (at the tank, not after the regulator).
For a "reasonable" amount of disposable gas cartridges (enough for the week, not a market stall) you don't need to notify them.
Compressed air: (in case you wanted to know)
Regular compressed air can be carried in certified/tested cylinders upto the rated test pressure on Norfolkline, without prior booking. Not allowed on the tunnel.