For Ratings, the minimum time to serve (MTS) = SITP + RoS, then + 12 months notice period.Are you sure it is complete your return of service, then 12 months notice? I'm pretty sure it's RoS including your notice period.
SITP's by branch are listed here: (BR3 Chapter 7, article 0759) http://www.royalnavy.mod.uk/~/media/royal navy responsive/documents/reference library/br 3/Br3book/ch07.pdf
The RoS criteria is outlined here (BR3 Chapter 53, article 5302, paragraph B)
BR3 said:Ratings and Other Ranks
(1) All new recruits enlisting in the Naval Service, who have not previously so enlisted, have a statutory right to claim discharge within their first six months of service, provided that they have completed a minimum four weeks’ service (excluding leave periods and any forfeited service) from their date of entry, or until the day before their 18th birthday, if this comes after the end of the six month period.
(2) For ratings and other ranks, a ROS is required to consolidate an individual’s training and give a reasonable return on the investment made by the Service.
Having completed six months’ service, or reached the age of 18 (if this comes after the end of the six month period), and subject to the exceptions that follow, ratings and other ranks become eligible to submit an application for Early Termination on completion of a minimum of 2½ years’ service – a period commonly known as the minimum time to serve (MTS) – from the end of their Standard Initial Training Period (SITP). The exceptions to the 2½ year rule are Student Naval Nurses and Medical Technicians who must complete a minimum of 3 and 3½ years respectively from the end of the SITP before they are eligible to apply for Early Termination.
(3) The SITP varies across specialisations and is the theoretical aggregate of the time that a rating or other rank spends undergoing the various components of initial training, assuming a smooth flow through the system and including an element for leave. Dates of actual completion of training should not be used. It should be noted that, if the length of an SITP is reduced, ratings and other ranks within the respective specialisation(s) will also be able to use the reduced period in the calculation of the earliest date on which they can submit their notice. If a SITP is lengthened, ratings and other ranks will retain their shorter SITP period.
(4) Unlike the ROS for further career training, the MTS period cannot run concurrently with the notice period. An individual must complete the MTS period before they become eligible to apply for Early Termination. http://www.royalnavy.mod.uk/~/media/royal navy responsive/documents/reference library/br 3/Br3book/ch53.pdf