So if the burst disk has been removed, royal mail or any courier service for that matter will be more than happy to post the air tank?
Removal of the burst disk guarantees it's not pressurized (note there are 2 burst disks - high pressure for the cylinder, low pressure for the regulator)
Removal of the regulator makes cylinders comply with customs for international travel so they know it is depressurised and can look inside
* do be aware of thread damage, whether or not the regulator was loctited in place, and reassemble to the correct torque rating, and don't mess about with cylinders if you don't know what you are doing *
Removal of either may or may not comply with the carriers policy
Check first, and don't just ask the person at the counter - if you do then take their details for later on in case they were wrong