Mellow Yellow
Have you been earning over the threshold the past 2 years? If not, then just tell them this. If you have been then just play dumb, say you didn't know how to go about repaying whilst you were abroad etc but now have all intentions of repaying the amount you should be. I guess they might make you pay back what you should have done over the past 2 years, but they might well agree to an affordable repayment plan. When I first started paying back my student loan the repayments were nothing anyway, about £20 per month, it's only once you're earning quite a bit that the repayments rack up.
I think it's very unlikely they'll demand repayment in full immediately (unless they think you have the means to repay in full now, and if you do why not repay it now anyway and save the interest?). I doubt the student loans company will go down the route of criminalising people in this situation, ultimately what they want is the money back, and throwing people in jail is not the way to do this, especially those who've shown a willingness to repay.
FYI in the UK if you're paying it back by PAYE your repayments are only applied to the loan annually. I'm sure you'll be able to make an annual payment.
If I were you I'd contact them and ask your other questions.