For £5 an extinguisher most places include service parts as required by the BS Basic Service (O-rings, tags, etc).
Some will charge separately for everything, even pumping the gauge and putting a new dot on.
In my Opinion if you are carrying out a Basic Service then the fee should include the consumables that must be changed as part of the service. Extras like a new metal pin if the original is missing, recharges, valves, hoses, nozzles, bases are fair game extras.
Because of the Far & Middle East and the Internet the extinguisher industry is becoming deskilled, disposable and hard to make a living in as it is as cheap to buy new extinguishers every few years rather than the workshop actions that used to keep stuff in service for over 20 years.
As for the annoying Chubb, Gloria & Ceo-Deux plastic OK pins, they are no issue for most firms as if intact they will just plastic tag them and change the tag each year (despite the use of the tag being against manufacturers instructions, invalidates the extinguishers EN3 compliance and doesn't follow all the basic service required actions) and if it's broken they will re-use it with the handy plastic tag again!