By acknowledgement - this means that you will of had to either paid towards the debt or put it in writing that you acknowledge the debt in the last 6 years. If you are sure you havent, then the debt is legally statue barred - meaning it cannot be enforced in court. That doesnt mean they aren't free to pursue the debt, they just cant use the courts to enforce it (so they can only hound you by phone and letter etc!). But if you do pay or acknowledge it now, the clock will start again I believe.
Most of the debts will probably now belong to Debt Collection Agencys. You could put it in writing, if they contact you, that you do not acknowledge the debt, & even if it was yours you believe it to be statue barred. Then send a CCA request asking them to prove the debt is yours by sending a true signed copy of the agreement. They most probably cant, and may well back off altogether at that stage - or at least offer you a much reduced settlement figure. You can see
www.debtquestions.co.uk or
www.consumeractiongroup.com for more info on this.
Morally speaking, if you know the debt is yours & the balances are what you expect you should still pay. You could try the above approach and offer to settle at a reduced rate (not by acknowledging the debt, but as a goodwill gesture - put it in writing that any such offer is just that). Some types of debt cant be statue barred, though.