Is it possible to apply for Thai ID card without my mother present? I’m half Thai but only hold a UK passport and wanting to stay long term in Phuket 

Born in the UK?  Already have a Thai birth certificate from the embassy?  This website may be of use to you;

https://www.thaicitizenship.com/thai-citizenship-when-born-overseas/

You havnt stated your age, but in any event you will need a copy of the Tabien bahn where you are staying or of the one you are registered in. Also a copy, (some will want origina)l birth certificate.

23 hours ago, T Matts said:

Is it possible to apply for Thai ID card without my mother present? I’m half Thai but only hold a UK passport and wanting to stay long term in Phuket 

To my knowledge you need to be registered in a Thai House Book to be eligible for a Thai ID-card; i.e. living/resident in Thailand.

 

You probably needs to be mature – i.e., 20 years or older – to apply for anything without parents or a guardians acceptance; I'm not sure of there is a lower age for ID-cards.

 

You don't need a Thai ID-card to staying long term in Phuket. You needs either a long-stay visa and extension of stay when using your British passport, or get a Thai passport. With the latter you enters Thailand as a Thai national and there is no limitation of the period you can stay in the kingdom.