I also contacted Sony about the code and student cashback, this was the feedback:
Retailer discounts are usually compatible with Student Cashback.

I have just been able to check it and according to the description under "current" it says how the voucher can be redeemed. This voucher is entered and deducted during the order process on our site at the virtual checkout. I have checked the information on the Sony website, unfortunately I cannot guarantee it. In this case, I would recommend that you contact Sony directly, as the "student" cashback is processed directly via Sony. It can be combined with the cashback campaigns advertised on our website.