There is a difference between a failure to pay a Court fine (which this appears to be) and failure to pay a contractual debt (for example, to a lender).
In the latter case, recent case law and enactment of the European Convention on Human Rights into domestic law in 2003 incorporated the right not to be imprisoned for failure to pay a contractual debt. Consequently, the law was changed so that when creditor applies to have debtor imprisoned for non-payment (s)he must prove 'wilful and culpable' non-payment. This level of proof does not exist for failure to pay court imposed fine. However, this level of proof has rendered imprisonment for contractual debt very difficult to achieve in practice.