Nationals of the United Kingdom and over 40 other countries are eligible to travel to Thailand, for tourism purpose, with the exemption of visa and are permitted to stay in Thailand for a period of not exceeding 30 days. Therefore, you do not need a visa.

However, please make sure that you are in possession of a passport valid for at least 6 months, a round-trip air ticket, and adequate finances equivalent to at least 20,000 Baht (450 pounds) per person or 40,000 Baht (900 pounds) per family. Otherwise, you maybe inconvenienced upon entry into the country.

Furthermore, foreigners who enter Thailand under this Tourist Visa Exemption Scheme may re-enter and stay in Thailand for a cumulative duration of stay of not exceeding 90 days within any 6-month period from the date of first entry.

The list of the 49 countries could be found at the Ministry of Foreign Affairs of Thailand’s website (www.consular.go.th) or (www.thaiembassy.org/london)

 You can apply for a 1-year or 3-year multiple-entry business visa which would allow  you to travel to Thailand as frequently as you want while the visa remains valid, and you would be permitted to stay for a period of not exceeding 90 days on each visit.    

           
You can find more information on the basic requirements for visa application at www.consular.go.th. To ensure that your application will be conveniently processed kindly check with the Royal Thai Embassy at www.thaiembassy.org/london. 
 

In order to work in Thailand, there are 2 important elements:

(1) Non-Immigrant “B” Visa and

(2) Work Permit

First of all, you need to have a prospective employer in Thailand. Then you may apply for a Non-Immigrant Visa category “B” (business) at the Royal Thai Embassy in London or the Royal Thai Consulates in the United Kingdom using the basic required documents and recommendation letter from your employer along with your company’s business certifications as suggested in the website (www.consular.go.th).

Once you have obtained such visa, you can enter Thailand and therefore apply for a Work Permit from the Department of Employment, Ministry of Labour. In order to expedite visa issuance, your employer may, on your behalf, apply for the Work Permit in advance (with the form called Tor Thor 3). The Department of Employment will then issue a pre-approved certificate to be used in your visa application. In such case, please make sure to obtain the Work Permit (Tor Thor 2) once you arrive in Thailand.

You have three options:

(1) Lithuanian nationals are eligible to apply for visa on arrival (VOA) at any of 26 designated checkpoints in Thailand, including, of course, all international airports.    

 
Currently nationals of 26 countries are eligible under this scheme. You can see the list of such countries in the website (www.consular.go.th) or (www.thaiembassy.org/london)

With the Visa on Arrival, you would be granted a stay of a period of not exceeding 15 days. But you must have a passport valid for at least 6 months, a round-trip air ticket where date of departure from Thailand is within 15 days of the date of entry, and adequate finances equivalent to at least 10,000 baht (250 pounds) per person.

(2) There is a Royal Thai Honorary Consulate-General in Vilnius, the capital of Lithuania. You may apply for a Thai tourist visa there. In such case, you would be permitted to stay in Thailand for 30 days. 

(3) You may also apply for Thai tourist visa at the Royal Thai Embassy in Moscow or at the nearest Royal Thai Embassy which would also permit you to stay for 30 days.

 In this case, please bring your passport to the Visa Division, Department of Consular Affairs, Ministry of Foreign Affairs, on Chaengwattana Road in Bangkok. If the visa is genuine, you can obtain any corrections or amendments of visa there.

Although Indonesians do not require a visa when entering Thailand for tourism purpose if their duration of stay in Thailand does not exceed 30 days, however, as you plan to attend a training course in Thailand, it is recommended that you apply for a Non-Immigrant Visa “B” for your trip to Thailand.
You may apply for such visa at the Royal Thai Embassy in London by providing basic documents which include a recommendation letter from your company verifying your status and the assignment. You will also need documents certifying that you are legally employed in the United Kingdom and confirming that you will be able to re-enter the United Kingdom without any problems.       

 You may apply for an extension of stay at any Immigration Office in Thailand (www.immigration.go.th) where you may be permitted to stay for another 30-day period. You must do so before your stay permits expires, otherwise you will be fined 500 Baht for each day you overstay in Thailand.    

You need to apply for a change of visa status, and apply for a Work Permit.
You are therefore recommended to consult with the Immigration Office (www.immigration.go.th) and the Department of Employment (www.doe.go.th) accordingly.

 Although you will work as a volunteer, you do need a Non-Immigrant visa as well as the Work Permit. The NGO must be legally registered with the Thai authority, and that you need a recommendation letter from the NGO for your visa application and the Work Permit. 

That is not a correct understanding. The validity of visa and the duration of stay are not the same. The 3-month validity of the visa means that you must use the visa within 3 months from the date of issue. The duration of stay is the period in which you are permitted to stay in Thailand granted by the Immigration Officer once you arrive in Thailand, that is, up to 60 days from the first date of entry. You can learn about the visa validity at www.thaiembassy.org/london.

 Before you leave Thailand for Singapore, you may apply for a re-entry permit from the Immigration Office. You are able to do that at any International Airport too.
 The re-entry permit will allow you to enter Thailand again before the end of next month. The permit will also enable you to stay until the end of next month, unless you get an extension of stay from the Immigration Office.

Yes, there is special visa called Non-immigrant “O-A” (Long Stay). Foreigners who are 50 years of age or older who wish to stay in Thailand for a long period and do not have any intention to work in Thailand may, in case of British national, apply for such visa at the Royal Thai Embassy in London ONLY. The holder of “O-A” visa is allowed to stay in Thailand for 1 year from the date of first entry, and is also able to apply for an extension of stay afterwards. Please see the qualifications and requirements for application and relevant information in the website (www.consular.go.th) or (www.thaiembassy.org/london)