as per UK consulate website in US
http://www.britainusa.com/visas/yestransit.asp
The following countries require a Direct Airside Transit Visa to transit the United Kingdom. Even if you only intend to remain airside and will not pass through UK Immigration.
AFGHANISTAN, ALBANIA, ALGERIA, ANGOLA, BANGLADESH, BELARUS, BURMA, BURUNDI, CAMEROON, PEOPLES REPUBLIC OF CHINA, COLOMBIA, ECUADOR, ERITREA, ETHIOPIA, GAMBIA, GHANA, INDIA, IRAN, IRAQ, IVORY COAST, LEBANON, LIBERIA, MACEDONIA, MOLDOVA, NEPAL, NIGERIA, PAKISTAN, PALESTINIAN AUTHORITY, RWANDA, SENEGAL, SIERRA LEONE, SOMALIA, SRI LANKA, SUDAN, TURKEY, TRNC, UGANDA, VIETNAM, YUGOSLAVIA, ZAIRE (formerly Republic of Congo), ZIMBABWE.
Exemptions: You are exempt from this requirement if:
You hold a valid US or Canadian visas who also hold a ticket to or from the US or Canada routed via the UK. This will include a period of 48 hours after the expiry of the US or Canadian visa for those leaving the US or Canada to allow for those who choose to leave at the very end of the validity of their visa
You hold a USA Permanent Resident Card (Green Card) issued on or after 21 April 1998.
You hold a Canadian Permanent Resident Card issued on or after 28 June 2002
You hold a valid Category D visa (printed on a red sticker) issued by EU/EEA member states, which are valid for more than 3 months.
You hold a valid EU residence permit
You hold a diplomatic or service passport issued by the Peoples Republic of China
You hold a diplomatic or official passport issued by the Government of India
If you qualify from one of the above exemptions you are not required to hold a Direct Airside Transit visa and may still benefit from the Transit without Visa concession.
If you do not qualify from one of the above exemptions you should complete form VAF 3 and submit the following documents to your nearest entry clearance issuing post;
If you intend to change airports in the United Kingdom you will require a visit visa. VAF 1