There are different rules whether you are a visa national or not. Those who are non-visa national, are not required to have a visa to transit through the United Kingdom on their way to another country. On contrast it may be a requirement for visa nationals to have a visa whilst on transit.

However this may be waived if the Immigration Officer is happy that you are travelling to a country outside the common travel area (CTA), ensuring that you arrive and leave by air within 24 hours and you can continue to another country where you will be guaranteed entry. If you arrive at a United Kingdom port on a ship you must also leave on the same ship within 24 hours to be able to transit without a visa.

Please check the list of countries that are not covered by the transit without visa concession, which is always updated on the Home Office website. If your country is not covered by the transit without visa concession you will need to apply for a direct airside transit visa or a visitor in transit visa if you wish to transit through the United Kingdom.