The 6 years to reclaim unsecured debt (12 for mortgage debt) begins from when you last admitted owing the money or made a payment and the time restarts with each communication you have with the creditor - as per The Limitation Act 1980 (for England & Wales) and The Northern Ireland Limitations Order 1989. Once the time limit has expired the debt becomes unenforceable or "statute barred".
Similarly, under The Prescription and Limitation (Scotland) Act 1973 it is 5 years for unsecured debt (and the interest part of a mortgage) but 20 years to recover the capital part of a mortgage.