STOP!
If the debt is an old debt, then you need to be aware of the Limitation Act 1980 Section 5.
If, for a period of 6 years or over (for unsecured debts), no payment of the debt has been made, and you have not acknowledged the debt in writing, then the debt becomes what is known as "statute barred", meaning that the creditor/debt collection agency becomes unable to take legal action against you to recover the debt.
Check the following link, which explains this legislation in more detail, http://www.nationaldebtline.co.uk/england_wales/factsheet.php?page=25_liability_for_debts_and_the_limitation_act
You need to be very careful in your dealings with the debt collection agency. There is a sample letter available here, http://www.nationaldebtline.co.uk/england_wales/factsheet.php?page=25_liability_for_debts_and_the_limitation_act#h
As long as you have not acknowledged the debt in writing or made any payments to the debt collection agency, and any written acknowledgement of the debt or payment was made over 6 years ago, then the debt would still be "statute barred" - you would thus be able to write to the debt collection agency using a variant of the sample letter above stating the statute barred legislation - and if the debt collection agency continued to pursue you over the debt, then you would be able to claim harassment contrary to Section 40(1) of the Administration of Justice Act 1970.
Defaults on your credit file should disappear after 6 years, and I believe it is not allowed to record a default on a credit file for a debt which has since become statute barred.
There are also specific guidelines by which debt collection agencies must operate which are laid down by the Office of Fair Trading, please see the following link for more details - http://www.oft.gov.uk/advice_and_resources/resource_base/legal/cca/debt-collection
It is unlikely that Barclays will gain from you repaying the debt to the debt collection agency. The standard model by which many debt collection agencies operate is that they buy debts from creditors (e.g. Barclays) in bulk, usually at a highly discounted price relative to the magnitude of the original debt. When this is done, then Barclays would have essentially "written the debt off". The debt would become the "property" of the debt collection agency, and any payments you subsequently make to the debt collection agency would go towards the profits of the debt collection agency, and not to Barclays.