Sources of Authenticated Official information about carrying camping gas canisters on UK public transport

• UK National Rail Conditions of Carriage

49. Restrictions Any Train Company may refuse to accept an item of luggage, an article, an animal or a cycle, even though it meets the
    requirements set out in Condition 47 and 48 and Appendix B, if, in
    the opinion of its staff:
    (a) it may cause injury, inconvenience
    or a nuisance or it may cause damage to property;
    (b) there is
    not enough room for it;
    (c) the loading or unloading may cause delay
    to trains; or
    (d) it is not carried or packaged in a suitable manner

• Railway Bylaws

2. Potentially dangerous items
   (1) Except with written permission from an Operator or an authorised person, no
   person shall bring with him or allow to remain on the railway any item which,
   in the opinion of an authorised person, may threaten, annoy, soil or damage
   any person or any property.

So you’re fine taking a small camping-gas canister on a train, so long as it’s not dangerous (e.g. it might be dangerous if it’s leaking gas).

By the way, UK railway officials don’t in any case have a right to search you or your property. If that needed to be done, and you didn’t give them consent to do it, then they would need to call a police officer (might be the transport police, but they’re proper police officers, not railway employees). The only situation in which I can envision that happening would be if you’re mistaken for a terrorist suspect.

• The bus companies each have their own ‘conditions of carriage’ – google that term plus the bus company name and you’ll find them. They’ll all be pretty similar.

• Transport for London Conditions of Carriage (covers London Underground aka the Tube)

13.1.2 You may bring with you: …

• any other item, provided it is not dangerous or likely to injure anyone.

13.1.3 You must not bring with you anything that:
• is more than 2 metres long
• you are unable to carry yourself (including up/down
fixed stairways)
• is a hazardous or inflammable substance
• is likely
to cause injury or offence to other customers or to our staff
• is
likely to cause damage to buses, Underground trains or stations.

Although a gas canister could be considered a ‘hazardous or inflammable substance’, in my opinion you’re probably OK, so long as it isn’t leaky or damaged.

• Transport for London Railway Bylaws

2. Potentially dangerous items
   (1) Except with written permission from the Operator or an authorised person, no
   person shall bring with him, attempt to bring with him or allow to remain on the
   railway any potentially dangerous item.
   (2) A potentially dangerous item is an item which, in the reasonable opinion of an
   authorised person, may or may be used to threaten, annoy, soil or damage any
   person or any property. For the avoidance of doubt, a potentially dangerous item
   may include, but is not limited to:
   (i) a loaded weapon of any kind;
   (ii) any flammable, explosive or corrosive substance; and
   (iii) any item which is or may become dangerous.

Again gas could be considered a ‘flammable substance’, but in my opinion you’re probably OK as long as the canister is in good condition.

By the way, it’s super-rare for any of these bylaws or Conditions of Carriage to even be mentioned in normal life, e.g. it doesn’t occur to anyone to refer to them when a passenger tries to get their horse on a bus. Most people don’t know they exist, I think.

In general: Any item can be illegal if there’s reason to think that you are planning to use it as a weapon or to commit a crime.

Short answer: For a small undamaged canister as part of a set of camping equipment when you’re going on a camping holiday, you have absolutely nothing to worry about.

If you want to double-check with the British Transport Police then you can contact them here or on [email protected]

Thank you to @Stephen Kennedy for mentioning the Channel Tunnel in comments.

Essentially you need to have the same expectations for Eurostar and the Channel Tunnel as you would for travelling by aircraft. When you book your ticket you are giving their staff permission to search you and your luggage. Expect your luggage to be x-rayed, searched and/or sniffer-dogged, and expect the conditions of carriage to be rigorously enforced in respect of anything flammable.

It would not be sensible to carry camping gas or fuel in any form on these services.

• Eurostar Conditions of Carriage

Appendix 1
The table below sets out articles which may not be taken on board Eurostar trains (carriage prohibited)
Note that this list is not exhaustive; passengers will not be allowed to keep articles with them on the journey if the item is inherently dangerous, is likely to cause property damage, or if there is reason to suspect the article may be used to commit an act of violence or threat.

CARRIAGE PROHIBITED

• Inflammable substances (eg lighter fuel, methylated spirits) except in minimal quantities.

• Butane gas.

(Appendix 1)

• Le Shuttle Conditions of Carriage
  (Le Shuttle is the vehicle train – carriage of dangerous materials is more complicated, presumably because some of the lorries carry cargoes which fall into this category).

Non-native English speakers may find it useful to note that ‘inflammable’ means the same thing as ‘flammable’: both words mean things which have a tendency to catch fire.