It's been almost 10 years since I went through Sleaford Tech, but I went through as a graduate. All the graduates were commissioned on entry to IOT and were Student Officers, effectively paid as Plt Offs. The non-grads were commissioned at the end of IOT and were paid a lower rate - probably more akin to an SAC's pay level, I didn't pay too much attention to how much they were on.
That was way back then, can't see it having changed, certainly in principle. If they want to attract high quality graduates to join up, they have to renumerate them accordingly otherwise they'll all go to the private sector.