Does the RN have a Royal Charter? I am not sure it does. The RN is quite different to the RAF in its formation and terms of service. In the RN no contract is signed by new officers (not sure about the ratings) nor an oath of allegiance given. Acceptance of the "Queen's Shilling" i.e. pay, is deemed sufficient enough "contract". Whereas in the RAF I think both a contract is signed and an oath given.
I think this is also why an RN officer never formally "retires" as such, he or she is placed on the retired list of officers but retain the entitlement to placing "Royal Navy" as a post nom, whereas the RAF have to put RAF(Retd).
RAF officers do not have a contract either, they are 'employed' under Royal prerogative. (Incidently airmen don't have a contract either, they are 'employed' under the
Royal Air Force Terms of Service Regulations.
The use of RAF(retd) has become prevelant though it is, technically, incorrect. (
Debretts Correct Form refers). RAF officers of flt lt and above having served for 3 years on the active list retain the ability to use their military rank on retirement (QR(RAF)3002(1)).