HM Revenue and Customs (HMRC) are not known for being lenient when it comes to taxpayers who fail to pay taxes, and mere inability to pay tax when due is only regarded as a 'reasonable excuse' (and therefore ground for avoiding penalties) when the inability is 'attributable to events outside the person's control'.
When a man with a portfolio of properties sold one, making a gain which led to a substantial tax liability, HMRC did not accept that he had a reasonable excuse for failing to pay the tax because he did not have the money to do so. The reason why he could not pay the tax was that his bankers (who had made sizeable loans to him) insisted that he remit all the sale proceeds to them to reduce his debt and refused to release their charge on the property unless he agreed to do so.
He appealed to the Tribunal, which agreed that his inability to pay the tax was indeed out of his control: he could not complete the sale if the bank did not release its charge and it would not release its charge unless it retained the whole of the sale proceeds. Accordingly, he had none of the profit on sale, on which the tax charge was based, to pay the tax and he had a reasonable excuse for non-payment. The penalty was quashed.