Head over to the British embassy with your passport (and letter stating that you need a certified Thai translation). Give them a call but if you do it be warned that a lot of the UK embassies charge money for advice.
You may want to also check the legalities with adoption with the British embassy. You do not want to suddenly find out your adoption was not legal by the UK and as a result the child will not be able to gain UK citizenship (which can be awkward if you choose to go back there and live with the child).
British Embassy
14 Wireless Road
Lumpini, Pathumwan
Bangkok 10330
Thailand
Tel: +66 (0) 2 305 8333
Fax: +66 (0) 2 253 7121
Notarial Services:
Monday - Thursday: 0800 - 1100 and 1300 - 1530
Friday: 0800 - 1200
The link goes to the British embassy in Bangkok and also provides a map with their location.
You are strongly advised to seek the help of the British Embassy in Bangkok.The translation will be expensive but you can be assured it will be done correctly and will probably help your case along.
there are loads of places on wireless road that can help you with all this, they mainly deal with weddings and stuff