GF sister has a daughter to british man who left her a few year back and now has no contact. The little girl was born in the uk and has a uk passport, which is due for renewal next year. any advise would be welcome. Thank you.

what is it you are looking for advice on? your op doesn't really ask any questions...

Sorry, Renewing her passport?

renewing an existing passport can be done without the father on the scene, I believe. The child is a British citizen in her own right & that has been acknowledged by the British Government when they issued the first passport. Just apply for a renewal using the forms on the web.

Link to renewing childs UK pasport

After getting the passport, I'd also enquire about how to get about getting child alimony. The child was born in the UK and is a British citizen. The Child Has Rights, one of them is to demand that the father pays up for what he has helped creating

The embassy won't be helpful of course, just make them shut up by telling them that it is the British child that is asking

There are plenty of groups helping out children from broken relationships. British father not paying child alimony for his British child wouldn't be a first one, the child living in Thailand may but the responsibility doesn't change for that