We are stuck in Phuket but would like to submit a 10 year UK visitor application to VFS IN BKK. She has had 3 previous visas so biometric info should be on file. Does anyone know if it’s possible to submit an application using a BKK agency?

I don't think you can.  The applicant's biometric details need to be taken each time an application is made.  You might get more responses if your post was in the 'Migration and visas to other countries' Forum.

This is the wrong forum. But to answer your question(s), the applicant must go to Bangkok (or another visa application centre, if open) to give biometrics. And, yes, you can use a Bangkok agency, or an agency based anywhere, even outside Thailand. The application is submitted online, but the applicant still needs to give biometrics in person.

You've already asked in the appropriate forum, and been given a correct answer.

 

I'll close this.