I think it is reasonable to change from capital and interest to interest only, making the payments affordable whilst the financial side of divorce is sorted out.
I am surprised that she didn''t ask you (it is a simple form and cost in general terms to change the repayment method - cost me £80).
I would add that my ex refused point blank to help with the mortgage, let me market the property for sale, or let me change to intererst only - he refused to sign for over 12 months saying that it would cost him.
I am sure that he was waiting for me to sign on his behalf as when we were together I always signed his signature.
Are you really sure it is worth making an issue of this it isn''t as if she is not paying the mortgage (and you aren''t) or that she has taken out a further loan on the house.
Also strangely I was watching the antiques roadshow the other day and they were talking about The Queens signature and Prince Charles and Diana.
It turns out that often members of staff sign on behalf of the royal family and the subject came up was that not fraud?
It is obviously something that is understood at a high level and the expert involved stated that it is not fraud to sign someones signature - I suppose it isn''t if you are not attempting to gain anything by way of deception.
The criteria for fraud as described in a book I have on Fraud Act 2006 is firstly the act and then whether it was done intentionally to disadvantage the other party.
Where you have joint accounts you need to contact the account holder by writing and let them know that you are in dispute and going through the process of financial separation.
You both need to be sensible and reasonable here - neither of you disadvantaging the other by way of conduct.
Reducing the mortgage to interest only does not on the face of things sound unreasonable to me unless she is on a huge income and there are loads of marital assets.
Cashing the ISA does sound wrong but there again you have not confirmed how long this has been going on, what the assets are, your income, whether there are children, and all the other details which are involved in a separation/divorce.
If you want to make sure things do not get messier than they are already you both need to co-operate and get things sorted.
In reality there is not much a court or anyone else can do if for one reason or another both parties disipate all their finances in a bitter war of attrition in divorce.