Apply for sole custody based on that the mother is not to be found and get a competent lawyer.

Second option is to go to the UK and apply for sole custody there and then try to get a Thai passport at the Thai embassy.

Mario,

the second option might be interesting but Im not from the UK and my country demands a consent letter from the missing parent for entry.

thanks.

I understood you want to live in the UK. If you settle there with your child, it will be the UK courts that have a say if you can get sole custody. Not the Thai court or the courts of your home country.