Can I try and summarise?
The shock is a dangerous item which they can’t transport.
However they’re quite capable of transporting it to Surrey to sell it, clearly that’s not actually true then.
Ah, but it contravenes their stated T&Cs
Well actually that’s not the case either. The T&Cs (http://www.royalmail.com/sites/default/files/RM_General_TermsConditions_Oct13_1.pdf) refer you to http://www.royalmail.com/prohibitedgoods which redirects to http://www.royalmail.com/personal/help-and-support/Tell-me-about-Prohibited-Goods. The wording in that is:
Gases
Gases that are compressed, liquefied or dissolved under pressure, permanent gases which cannot be liquefied at ambient temperatures, liquefied gases which become liquid under pressure at ambient temperatures, dissolved gases which are dissolved under pressure in a solvent.
1. All flammable compressed gases are prohibited e.g. blowlamps; butane; lighters and refills containing flammable liquid or gas; ethane; gas cylinders for camping stoves; hydrogen; methane and propane.
2. All toxic compressed gases are prohibited e.g. chlorine; fluorine etc.
3. All non-flammable compressed gases are prohibited e.g. air bags; scuba tanks, carbon dioxide; fire extinguishers; neon and nitrogen.
No mention there of empty gas containers – that’s the bit they’re making up. The bloke from RM dangerous goods says:
Because the product is charged by filling with compressed air and whether it is full or not it is deemed a prohibited item under the above guidelines.
Except the T&Cs don’t actually say that, so if they claim that you agreed to them, then that still doesn’t justify their actions. IANAL, but I’ve spent time recently interacting with barristers (including being cross-examined by one) and I am sure that one would completely shoot down any claim that their T&Cs say empty containers are prohibited. It simply doesn’t say that, and implying it isn’t good enough – the exact wording is important. Any catch all involving the bit at the bottom about reserving the right to refuse any other item is quite clearly an unreasonable term, as how is the customer supposed to guess what non-dangerous items they will deem come under that?
Finally I note that in the reply form the RM dangerous goods team he quotes T&Cs section 4.9.6 where it says:
we may deal with such item in our sole and absolute discretion (without incurring any liability whatsoever to you or your intended recipient) including destroying or otherwise disposing of such item in whole or in part, or returning the relevant item to you.
…note the last phrase – I can’t see any justification at all for them selling your goods rather than doing that.