Christ - the amount of poor advice on this thread is awful.
OP, regardless of my personal opinion on your actions, if your employers pursue a criminal justice route, then the police will ask you to attend voluntarily as you pose very little risk to the public.
Again, if Primark decide to pursue, then you will be interviewed under caution. Please do ensure you exercise your right to a lawyer. Do not do what many do and be interviewed without a lawyer. You will be offered one.
Will you go to prison? That all depends on the pre-sentence report. However, as a former prisoner myself (who did go to prison for a first offence of fraud) fraud by misrepresentation and obtaining money by deception is a a serious offence.
Make sure you instruct a lawyer for the interview. You will bailed to appear at court and you should instruct a solicitor for the hearing. Find a reputable defence lawyer. If you plead guilty, it may well be the Magistrates (because fraud is serious) commute to the Crown Court if the sentence guidelines are outside of the powers oF the magistrates.
Unlock Charity do some fantastic advice so worthwhile having a chat with them.
As for the 'criminal record' Yes, (I have one) I always disclose when asked even though mine are spent now. I have also managed to navigate the barriers with being open and transparent about my background to avoid employers having to go through the mill.
Please make sure you instruct a lawyer at the interview.
It may be that Primary decide not to pursue and end your employment under gross misconduct.
Unless of course, their investigation uncovers other matters, as you're currently suspended, I'd wait to hear from HR.
Good luck.