It does come under F-gas regulations, only trained and suitably qualified personnel are allowed to work on the system. However buying gas doesn't seem to be regulated... (Although 'handling refrigerants' is) If the system has lost gas then there will be a leak on the system, which will need to be properly repaired by a qualified person, yet some how anyone can just buy these pre-charged canisters to charge the system themselves which will just leak back out. Any person could walk into a refrigeration supplier and buy as much gas as they want, it's just a massive grey area in the regulations. On the other hand, engineers are getting £1k fines per bottle of reclaimed gas for not carrying filled out paperwork with them.
The weight of gas is for leak checking and maintenance, as vehicles gas charge falls under 3kg it doesn't need a regular inspection.
R134a is also being phased out.