Sorry, another SIM unlock on Android 4.3 question

I know there are many threads on this, but after spending hours looking through them and info on other forums, and then attempting a buy a code, I'm still unclear as to whether my SGSIII running 4.3 can by networked unlocked, and if so how?

The phone is locked to EE in the UK. I think this is a custom ROM, but very close to stock:
Baseband: I9300XXUGNA8
Kernel: 3.0.31-2429075
dpi@DELL135 #1#Wed Apr 30 18:49:01 KST 2014
Build number: Android Revolution HD 52.0 by mike 1986

Rooted by Chainfire SuperSU v1

I purchased a code online, but received an email 24hrs later saying

"Unfortunately we have been unable to calculate your unique unlock code at this time. This could have been down to a number of factors from the age of the handset to the code simply not being in our unlock code database or partners database.

You will be refunded in full."

Using the manual options, I've got as far as finding the service menu using the info here:

https://community.ee.co.uk/t5/Androi...ck/td-p/167059

But there is no PERSO SHA256 menu.

This has been reported by other users, but I'm unclear how to proceed from here, or if indeed this is a dead end and there's nothing else I can do?

Download from this link