I think now is the time to get proper legal advice about the whole situation and then atleast your mum would know her exact position.
If the credit cards were in your fathers name only, as unsecured debts, it would be difficult to take any enforcement action against his estate. BUT the law in this area is quite complicated, hence why it might be a good idea to get legal advice. There are provisions in the Insolvency Act 1986 section 421A to consider. ( I found this online, I am no expert). My reading of it, is that creditors can apply for something called an Insolvency Administration order, where someone is appointed to look into the deceaseds financial position. Because of the complicated nature of the various debts, this is not something that can be advised by experts or non experts online. Your mum needs someone legally qualified to look at all the various debts and to give her tailored advice.