You will definitely not go to prison. You won't even be prosecuted. But HMRC wil want you to make an honest declaration of your income, and they will want you to pay up. Firstly, Class 2 National Insurance, which will be about �250 a year plus a late notification penalty of (I think) �150. The tax and, if necessary, Class 4 National Insurance can't be worked out until you've done a tax return. They will ask you to pay the whole amount you owe right away but if you haven't got it you should pay as much as you possibly can and ask for time to pay the rest. HMRC aren't particularly keen on this and will want to know that you've done everything you can to get the money you owe but they do know that you can't get money from thin air and it's usually possible to come to some arrangement.
I'd see an accountant if I were you, even though it may seem like spending money you haven't got. What you spend will probably be a good investment.
As thugulike says on the jobs thread, this happens a lot and in some ways it's no big deal. But there's no getting away from the fact that for 2 years you haven't been paying your way and it's time to settle the bill that others have been paying while you haven't.
Whatever you do, don't let the bill get bigger by doing nothing - they'll find you eventually!