They are a large and ethical insurance company.
Whether you are liable for these extra costs depends on firstly the authorisation you were given which in my opinion, you can rely on and secondly, if, when you leave a company, the insurance company is allowed to leave you hanging without funding, in the middle of treatment.
In that respect, you would need to check the terms and conditions of the scheme in fine detail.
When you have checked the conditions of the scheme in fine detail, then if this is covered, that you know where you are, you are arguing with the insurance company making a formal complaint and if necessary referring the matter to the Financial Ombudsman Service. If however it’s not covered, you are still going to make a formal complaint to the insurance company because you relied on what you were told that you are still covered in if they still say that you’re not covered, then make the complaint, once again to the Financial Ombudsman Service.
Can I clarify anything for you?
Please don’t forget to rate the service positive. It’s an important part of the process by which experts get paid.