Not posted for an age but thought I’d add to this, although I’ve been out of branch for about 18 months so there may have been some recent changes. There is what could be called a ‘fast track’ route to SNCO, but you have to join as an ASOP and then apply once you’re in, you can’t join it direct from outside. Once you have (I think) two years’ experience as an ASOP you can apply for SNCO (FC) duties, which if successful will see you enrolled on the Weapons Controller course in your current rank alongside the other trainees (usually Officers out of IOT). If you pass the course and gain your Controller qualification, your Sergeant tapes are awarded at the same time. Due to SNCO (FC) having a separate promotion ladder to the rest of the ASOP trade onwards progression does seem to be quicker.
Note that this route is for Weapons Controller only, the Surveillance stream does not have a similar fast track route to SNCO. If you fail the Controller course as an ASOP you revert back to previous duty.