While I appreciate the position you are in, I think you are trying to address the situation in a way that isn't going to work for you.
The UK bank won't care. They have done their risk assessment and he is repaying them. He isn't paying anything towards your previous mortgage together and their is no court mandated maintenance so why would it be in their interests to do anything. Declaring the mortgage null & void, would involve them calling in the debt and paying costs of legal and estate agents when they have no benefit to doing so. Their priority is the repayment of their mortgage on line with the terms and conditions, not sure situation.
Similarly, the Irish Bank have a joint & several loan. They don't get involved in which of you is making the debt. Once it's being repaid, they have zero reason or motivation to do anything.
Your issues isn't a bank issue.
You need legal advice. Your contributions towards the mortgage will count in your divorce & the way your equity is split. Spend time with citizens advice and legal advisors. Strategic default may be the only way to sell the house before he wants to but again this is not an option to be taken without legal advice as it will impact your credit rating.
Talk to both MABS and Abhaile - free support organisations who assist with debt issues. I suspect they have seen many situations like yours but more important Citizens Advice, Free Legal Aid etc. The banks can't help sort your problem,only the law can.