From the careers website....
"Graduates entering as commissioned officers may be offered enhanced promotion and can expect higher salaries"
You do get a higher rate of pay during IOT (Well at least graduates used to!) to balance out the fact you have a student loan to pay off for example. You will also after graduation uni graduates tend to become a Fg Off compared to your straight out of school mates who will be APOs.
There is generally a pay breakdown in the bumpf from the AFCO. I don't have it to hand and what I do have it out of date and unlikely to be of much help, but generally after tax, non-graduates were on around £800/month and graduates approx £1000ish/month. This may have well changed with the new IOT as I believe they no longer have the Student Officer/Officer Cadet divides.
I stand to be corrected as I believe my info is out of date