Whilst transiting through the United Kingdom without passing through United Kingdom border control, and you do not qualify to transit the United Kingdom without a visa, you will need a direct airside transit visa (often called a DATV), unless the direct airside transit visa exemptions apply.

Restrictions

A direct airside transit visa does not entitle you to enter the United Kingdom and go past United Kingdom border control. If you want to enter the United Kingdom, you will need to apply for either a visitor in transit visa or a visit visa. Some transit travellers do not need a visa to pass through the United Kingdom on the way to another country.

Not Exempted

A direct airside transit visa is necessary if you are a national or citizen of one of the countries that are not covered by the transit without visa concession. However, certain nationals are exempt from needing a direct airside transit visa and certain other passengers do not qualify to transit using a direct airside transit visa and will need another type of visa.