"Degassing the marker with bottle on indicates using an on/ off asa. This leaves the regulator open and the asa pressurised"
No it doesn't If you degass using an eclipse ASA the button that presses on reg is not pressed in so the tank is loose on the marker and not pressurised,I can't see a problem with that as its exactly the same as taking it off.
I leave them on as much as possible as it saves wear on the thread and protects them,I then charge whilst on gun but not gassed up from a 300 bar bottle.
As a firearms instructor,most police officers would not even recognise a tourney marker as an air weapon .
They would be concerned about Milsim if you were waving it around inappropriately,most PCP air guns are not degased just unloaded and cased whilst transported.
Shotguns and section one firearms are just unloaded and cased,ammunition stored separately ,not left un attended.
Removing bolt is good practice if possible.
Transported in the locked boot of a car is not a public place.
I have a section 1 Tranquilliser rifle which can't be degased without loosing all the pressure from the bottle,so that's transported pressurised .its used for tranquillising dogs which have bitten and dangerous cattle rather than shooting them.
(b)a shot gun or an air weapon shall be deemed to be loaded if there is ammunition in the chamber or barrel or in any magazine or other device which is in such a position that the ammunition can be fed into the chamber or barrel by the manual or automatic operation of some part of the gun or weapon.
Have a read.
http://www.legislation.gov.uk/ukpga/1968/27
So come on sensible advice please.
