Debts only stay on your credit record for 6 years (statute of limitations act) and this could mean that you have debts that could never be enforced in court. These debts whilst they can be chased, are really only repayable by you on a voluntary basis.
Sometimes creditors may not have registered the debts on your credit record, but it is possible they might suddenly appear if a debt collection agency buys the debt and it is still enforceable.
Would suggest that you deal with any debt issues that are on your credit record and then see if any others come out of the woodwork. If any other debts do come to your attention, ask for proof of the debt and a copy of the creditors last statement showing the last payment made.
If the debts include old student loans, it depends on whether these are pre 1998. Before 1998, they are subject to the Consumer Credit Act and may not be enforceable after 6 years of non payment or ackowledgement by you. Student loans after 1998 can I believe still be chased, with payments deducted from wages.
You can also phone National Debtline for advice, as it is probably better for you to discuss this. Atleast you can ask specific questions on the debts that you have.