I'm no expert on pensions, but you should be entitled to at least your contributions plus income on the fund to date. That's because it would be under a trust in some form, held on your behalf.
As to what I do know a bit about: your plea of guilty at the first available opportunity helps you one heck of a lot. What you don't say is why you did it? Was it one of those cases where you saw a flaw in the system, took a small amount, thinking it was just that, didn't get detected, and did it again? And when you finally were detected, you were surprised how much the total was?
Your problem is that theft by employees is seen as pretty serious anyway. Theft by someone who has control of records, or the means of recording intake,or who is receiving money, is regarded as particularly serious. It means jail. However, you might just get away with a suspended sentence, if your circumstances and reason for succumbing are good enough . You need a solicitor who does crime and a barrister in due course.