I THINK MEMBERS OF THIS FORUM ARE MORE KNOWLEDGEABLE THAN THESE 10+2 FAIL TRAVEL AGENTS:
Let me explain transit Visa in your matter : Passing through the United Kingdom on your way to another country without staying in the United Kingdom.
In your case it is -Direct airside transit visa (DATV) : 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).
THE EXEMPTIONS ARE GRANTED TO :
You will be exempt from the DATV requirement and may be able to transit without visa if you hold one of the following.
A valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel through the UK as part of a journey from another country or territory to the country for which you have the entry visa.A valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel through the UK as part of a journey from the country for which you have the visa to another country or territory.
A valid airline ticket for travel through the UK as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, as long as you do not transit (pass through) the UK on a date more than six months after the date on which you last entered Australia, Canada, New Zealand or the United States of America with a valid visa for that country.
A valid USA I-551 permanent resident card issued on or after 21 April 1998.
A valid Canadian permanent resident card issued on or after 28 June 2002.
A valid common format category D visa for entry to an EEA state – see our 'EEA and Swiss nationals' leaflet for a list of EEA states.
A valid common format residence permit issued by an EEA state under Council Regulation (EC) number 1030/2002.
A diplomatic or service passport issued by the People's Republic of China.
A diplomatic or official passport issued by India.
A diplomatic or official passport issued by Vietnam.
SO IN UR CASE NO REQUIREMENT OF TRANSIT VISA

I am producing this information from the website of uk border agency,http://www.ukvisas.gov.uk/en/howtoapply/infs/inf20transit#Q7