If the child was born and registered in the UK, or born in Thailand and birth registered at the British embassy, OP could obtain a copy UK birth certificate online from the GRO: https://www.gov.uk/order-copy-birth-death-marriage-certificate
If OP has the father's DPOB, he could also obtain a copy birth certificate from the same source if required.
Procedure for obtaining a child's first passport here:
https://www.gov.uk/get-a-child-passport/first-child-passport