Changing Step daughter's Surname

[dubbplatz]

Hi,

     Right here it is my Thai wife moved to England with my Thai step daughter on  a settlement visa. I had to apply for the step daughters settlement visa in the surname of her real father thats on her Thai birth certificate. When she arrived in the U.K we changed the step daughters surname by deed poll into our surname and now the step daughter has Naturalized British citizenship. When i applied for her British Passport they wouldn't issue it in her new Surname as they are saying she is using two different surname and they won't issue it in her old surname either. They have said we need to change her surname in Thailand first or provide proof as to why she cannot change her surname in Thailand from the Amphur. Does anyone know how I go about changing her surname?and I we can't change it what proof do we need for the Passport office in the U.K Also my Wife has sole responsibility for my Step Daughter in Thailand and she has no contact with the father. Thanks 

[dubbplatz]

Also my Wife went to the Amphur in Khon Kaen they have said we need something( I don't know what from the British Embassy to show to them in the Amphur before they will change the step daughters surname to mine