"Graduate officer cadet" is not a rank. You'll be an officer cadet; you'll just be paid differently as a "thank you" for coming equipped with a degree as I said.
For the RAF, your rank is either officer cadet (if you don't have a degree) or student officer (if you do).
Officer cadets are paid as officer cadets: £36.92 per day. Upon graduation, they're commissioned as Acting Pilot Officers; £51.71 per day. Depending on branch, they spend a certain amount of time as an A/Plt Off before becoming a Plt Off; £58.36 per day. Generally, it's a year as an A/Plt Off and a year as a Plt Off. Then, you're a Fg Off, on £70.15 a day.
All of these pay rates are part of the combined Fg Off/Plt Off scale. Off Cdt is level 1, we don't use 2 or 3, A/Plt Off is level 4, Plt Off is level 5, Fg Off is level 6. After a year as a Fg Off, you move on to level 7; £72.00 per day.
After a set amount of time in Fg Off, you'll become a Flt Lt, and move to the Flt Lt payscales starting again at level 1; £89.89 per day. Again, on the anniversary of your promotion to Flt Lt, you'll move up a level.
Now, if you're a graduate, you technically count your commission from day 1 of IOT. On that day, you enter the Fg Off/Plt Off payscales at level 5; you're a Student Officer by rank, but you're being paid as a Plt Off.
On the old IOT, you graduated as a Plt Off, but 6 months after initial commissioning; ie a few days after the end of IOT; you become a Fg Off.
You didn't enter Fg Off at level 6 necessarily; depending on the exact type of degree you held, you may enter at a slightly higher pay point. When I went through, a masters degree got you 1 year less as a Fg Off, so you entered on level 8 while your mates went in on level 6. You did 18 months as a Fg Off with a masters, 30 months with a batchelors. Then you went to level 1 Flt Lt.
In branches like legal or engineering, experience can count for increased seniority (the term regarding how far up the payscale you are; ie a 3 year seniority Flt Lt will be on Flt Lt level 4) or accelerated promotion to Flt Lt. That's why it's possible for a chap with a MEng and 10 years working in industry to graduate as an engineering Flt Lt.
So there you are. Officer cadets start on level 1 of the Fg Off/Plt Off rates; level 4 after IOT; level 5 (with Plt Off rank) after a year; and up one level per year with promotions as required. After a certain amount of time (I think something like 3 years) as a Fg Off, they're off to Flt Lt level 1.
Student Officers start at level 5 (the Plt Off level; although they do not hold Plt Off rank yet); move to 6 (Fg Off; along with Fg Off rank) shortly after graduation, maybe higher depending on their degree; and do a fixed amount of time until Flt Lt level 1.
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To answer Gemma; you'll be paid £21 301pa on entry. Your degree entitles you to be paid as a Plt Off. On the old IOT certainly, you'd have stayed on this for 6 months, then moved to £25 605pa as a Fg Off. You'd spend 30 months as a Fg Off before being promoted to Flt Lt on £32 810pa.
You'd go up one level per year to a level 9 cap of £39 019pa, with smaller increases thereafter, competing for promotion to Sqn Ldr.