If you are planning on travelling to Thailand for 28 days or less and you have British, Dutch or USA passport you can get a Visa exemption on arrival. This is valid for 28 days and cannot be extended in Thailand. If you are planning on being in Thailand longer than 28 days you will need to pre-arrange a Visa. You can prearrange a visa for 60 days which you can extend to 90 days in Thailand.
There are two ways of arranging your visa. When you are from the UK. You can either go in person to the Thai embassy in London, or you can post your passport along with the necessary documentation and fee to one of the Thai Consulates. In general this works the same for the other countries like The Netherlands and United States too, but please contact the nearest Thai Consulate for visa enquiries.
Please note that you are responsible for arranging your own visa. Xtreme Gap cannot be held responsible if you fail to travel with the correct visa. So please check with your local embassy personally. The previous information is meant as a guide, but visa rules change often and frequently. So do not take this as the current law.