DATV nationals are visa nationals who must have a visa to pass through the UK on their way to another country, even if they are not entering the UK or changing airports. Unless they qualify for exemption from the DATV requirement, they cannot transit without visa (TWOV). (You can get more information about DATV exemptions below.)
Passengers exempt from the DATV requirement
Holders of certain documents are, regardless of nationality, exempt from the requirement to hold a Direct Airside Transit Visa when transiting the UK.
A transit passenger is not required to hold a transit visa if he holds, or a person with whom he arrives in the United Kingdom holds on his behalf:
1. a valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from another country or territory to the country for which the visa is held;
2. a valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from the country for which the visa is held to another country or territory;
3. a valid airline ticket for travel via the United Kingdom as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, provided that the transit passenger does not seek to transit the United Kingdom on a date more than six months from the date on which they last entered Australia, Canada, New Zealand or the United States of America with a valid visa for entry to that country;
4. a valid USA I-551 Permanent Resident Card issued on or after 21st April 1998;
5. a valid Canadian Permanent Resident Card issued on or after 28th June 2002;
6. a valid common format Category D visa for entry to an EEA State;
7. a valid common format residence permit issued by an EEA State pursuant to Council Regulation (EC) No. 1030/2002;
8. a diplomatic or service passport issued by the People’s Republic of China; or
9. a diplomatic or official passport issued by India; or
10. a diplomatic or official passport issued by Vietnam.
DATV nationals are visa nationals who must have a visa to pass through the UK on their way to another country, even if they are not entering the UK or changing airports. Unless they qualify for exemption from the DATV requirement, they cannot transit without visa (TWOV). (You can get more information about DATV exemptions below.)
Passengers exempt from the DATV requirement
Holders of certain documents are, regardless of nationality, exempt from the requirement to hold a Direct Airside Transit Visa when transiting the UK.
A transit passenger is not required to hold a transit visa if he holds, or a person with whom he arrives in the United Kingdom holds on his behalf:
1. a valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from another country or territory to the country for which the visa is held;
2. a valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from the country for which the visa is held to another country or territory;
3. a valid airline ticket for travel via the United Kingdom as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, provided that the transit passenger does not seek to transit the United Kingdom on a date more than six months from the date on which they last entered Australia, Canada, New Zealand or the United States of America with a valid visa for entry to that country;
4. a valid USA I-551 Permanent Resident Card issued on or after 21st April 1998;
5. a valid Canadian Permanent Resident Card issued on or after 28th June 2002;
6. a valid common format Category D visa for entry to an EEA State;
7. a valid common format residence permit issued by an EEA State pursuant to Council Regulation (EC) No. 1030/2002;
8. a diplomatic or service passport issued by the People’s Republic of China; or
9. a diplomatic or official passport issued by India; or
10. a diplomatic or official passport issued by Vietnam.
http://www.ukvisas.gov.uk/en/howtoapply/infs/inf20transit
http://www.ukvisas.gov.uk/en/doineedvisa/visadatvnationals