You asked for advice.
In your shoes I would have your UK solicitor contact your Embassy in Thailand requesting advice on a reliable Thai solicitor to handle your case in the Thailand courts.
Remove yourself as far as you can from the situation. All communications concerning this affair should be through your UK solicitor. Your Thai solicitor should report to your UK solicitor. Yes, the will be a large expense.
Try to forget about the issue and remember the good things concerning your father.
Legal issues are no win situations. Let the lawyers (solicitors) do there jobs. Try to minimize the impact of losing your father on you and your family.