Regional, Branch, Or Representative Office?

If any one has experience in this area I would appreciate the clearing up of my confusion.

I work for a UK registered organization which also has offices in the US and Australia as well as works on projects in nearly 40 countries around the world. We are looking to start a new volunteer project in Krabi. Volunteers come out to learn how to teach English while simulataneously offering free English classes to the community. The object of the free English classes is to assist the community with gaining English skills for jobs and encouraging sustainable development locally. Volunteers coming out to teach register and pay throught the UK head office. No money is accepted within Thailand. The head office will provide funds for the project and office in Thailand to function and support volunteers, staff, and teaching within the community. I am trying to find out the easiest and best way to register so that I can obtain visas for staff and work in knowledge of the Thai government. I have been doing some research and it seems that by working as a regional, branch or representative office are all possibilities but I'm not quite clear on which one fits our role.

Any suggestions?