OP this story makes me so sad. Do not let her bring the child here, please . If the child is settled in the UK, leave her to live her life there. You will not have any rights here in LOS; however hard you try. The child will not thank either of you if she is living and so called educated here in LOS. I know of what I say. My Thai Brit daughter, 24 year old now, is in the UK, fully educated there and happy to stay there. Despite all we tried to do, and her mother and I are as one on this, we had horrendous problems with me being recognized her biological Father here.
Do PM me and I will share the details of all we went through. Just don't let the child out of the UK until you have the full facts, if you ever do. Be well prepared to get a restraining order to prevent the child leaving the UK. Please do all you can to keep her there. Your relationship with the Mother is immaterial to the life and later wishes of the child. As an aside, our daughter dislikes Thailand and only comes here on sufferance. Let her make her own decisions when she is 18. By the way, a few million Baht will be needed for lawyers/court fees if you go down the route of trying to get recognition here.