"...do these cylinders constitute dangerous pressure receptacles in this context ?"
Yes, but exemptions exist for private individuals.
The Carriage Regulations are the implementation framework for the variously updated ADR provisions for the transport of dangerous substances by road as applicable within working environments. ADR exempts completely private, non-work related carriage subject to some conditions.
The goods must be packaged for retail sale and be intended for personal or domestic use or for leisure or sporting activities provided that measures have been taken to prevent any leakage of the contents during normal carriage.
Some classes of dangerous substances have specified limitation of quantities to qualify for this exemption, e.g.
Explosives (gunpowder/black powder) - 30 Kg
Flammable liquids in refillable containers - 60 litres per receptacle up to 240 litres maximum carry.
Other legislation may limit carriage of petrol etc.