The Kingdom of Thailand is a country in Southeast Asia. It borders Laos and Burmaa, Cambodia to the east, Gulf of Thailand and Malaysia to the South and Andaman Sea and Burma to the West. Thailand is a heaven for Visitors and Tourists and everyone must visit this country.
Fortunately, all the UK Nationals do not require any kind of visa in order to visit Thailand. However, this facility is only for people who want to visit Thailand for a period of 30 days with a possible extension of 10 more days.
Since you want to stay in Thailand for only 3 weeks or 21 days, you would not require a visa. You just need a valid passport, proof of sufficient funds, return or through ticket etc. You do not need to have any vaccinations etc except for situations where you are coming from African countries or countries with yellow fever.