in Thailand you will need a will for the courts and once they are happy with everything they will send the paperwork to your lawyer to help you with the bank because they will not give up the money until the court process has been done and you have the paperwork from the court
if you have any money available or property in England then you may need a will for the courts in England, the best thing to do is ring a solicitor in England to find out if they will accept any will you have written in Thailand
I've been down this road ok so it's up to you but you have to write a will you can do it yourself without a lawyer once you have got it done then you have to have 2 witnesses to sign the document then you will need to take it to a lawyer to look after it and he will be able to help her in court to ensure that she will have the money, not the thai government it's going to take about 1 year before she gets the money unfortunately it's how the system works in Thailand, make sure that you have two copies of the will one for the lawyer and one for the lady