Travelling with a gas bottle might seem a scary prospect when you're heading to the campsite, but the truth is that it's actually very safe and easy provided you follow some simple, common sense advice.
Travelling by car
You should not need any sort of sign on your car to signal you're transporting gas if you're using most common domestic sized gas bottles, but it's important that you remember that the bottle itself should be stored upright and as securely as possible. If it's rattling and shaking, or you can hear it while driving, you need to secure it tighter.
Then, it sounds obvious, but make sure that the bottle is off. With Calor gas bottles, for example, it is very easy to ensure there is no gas flow through the pipe. If at all possible, store the bottle in a trailer or caravan you're towing, though it is safe to carry the bottle in the car boot if necessary, provided it's properly secured.
A top tip is to keep the back windows rolled down a little to provide some ventilation. It shouldn't be a problem, but in the unlikely event of a leak, you're going to want some clean airflow as soon as possible.
Use your common sense and avoid leaving the gas bottle in direct sunlight, or in a hot car. The temperature extreme for gas to ignite in the bottle is likely going to be higher than you could ever sit in the car comfortably anyway, but don't leave your vehicle parked in the sun with the bottle in the boot.
Whether it's by rail or by ferry, you should have no problems travelling with a gas bottle, as long as it's secured in the aforementioned ways. All carriers will have individual weight limits, and while it's unlikely for an average camping trip you'll be carrying a bottle in excess of that weight it's always best to know ahead of time.
Then always make sure that you declare your gas bottle whenever you're boarding, they may want to inspect the bottle and how you're carrying it. Now you're free to camp!