Maybe a silly question, is this a concern for insurance reasons or because they scan parcels (do they?) and it might be returned?
It's the law:
Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009 (CDG Regs)
Accord européen relatif au transport international des marchandises dangereuses par route, known as ADR
EU Transportable Pressure Equipment Directive.
Any carrier has to comply with the above laws and directives. When you post an item it depends on the method used as to where it fits into the regulations, and the carrier may have a choice of options to carry (and ensure that the method you buy is carried by land/air/sea legally) or their systems may not guarantee your package doesn't go via an illegal or dangerous method
Royal Mail do X-ray everything, so you're more likely to get caught doing it the wrong way - and have goods destroyed, get a fine etc
Insurance affects the outcome when a package goes missing, but does not protect against using a prohibited method
Blame the people who spent years avoiding declarations by just saying 'sporting goods' then getting caught posting pressurised air cylinders get ended up in air mail
(Even inland)