First things first, and to set your mind at ease, you are not going to go to prison. Total value of this sounds like roughly 5,000 pounds of benefit fraud (2 years of IS at approx 120 every fortnight and 2 years of approx 1,000 per year council tax) and, if you have no previous convictions, you wouldn't be going to prison.
When you attend the interview, I strongly suggest you arrange to get a local solicitor to attend with you - they can be paid under the legal aid scheme so it shouldn't cost you a penny and it is well worth having someone by your side who understands exactly what's going on and can advise accordingly.
In terms of what might happen after the interview, there are number of options. The DWP/local authority can take no action at all, give you a formal warning (just noted on your account with the relevant benefits agency in case of any further irregularities), administer a penalty (30% payment on top of what was received fraudulently - call it a fine without going to court) or, in the worst case scenario, issue a summons to court.
Even in the worst case scenario of being summoned to court, you will not be going to prison. Max sentence I'd envisage for something like this would be a community order with requirement to undertake 100 hours of unpaid work - like the old community service.
It's perfectly possible that this will not go to court so don't get yourself too worried about it.
You will be required to repay the money in any event. Don’t be surprised if you receive a letter from the DWP/local authority debt collection department seeking immediate repayment. If you do get this letter, contact them immediately to arrange payment, lump sum if you can, or by installments.
Hope that assists but please feel free to ask any follow-up questions that occur.