From murhty.com hope this is helpful to you
We have recently learned of instances in which U.S. permanent residents of Indian origin and Indian citizens with nonimmigrant visas wishing to visit the U.S. were unable to board flights from India to the United States via the United Kingdom (U.K.). This was due to these individuals' failure to have proper transit documents. This was a surprise to many who had assumed that, if they were not leaving the airport and were merely in transit from one airline to another, there was no requirement to obtain a visa for the U.K. Citizens of India, however, must have a Direct Airside Transit Visa to travel through the U.K. This includes permanent residents of the U.S., if the person's plastic green card (I-551) was issued before April 21, 1998.
In addition to citizens of India, this requirement applies to nationals of 22 other countries, including: Pakistan, Nepal, Bangladesh, Burma, China, Columbia, Iran, Iraq, Ivory Coast, Vietnam, Nigeria, and Ethiopia. If any such citizen intends to change airplanes in the U.K., a visa will be required for her/him to enter. There are a number of exemptions from the need for a Direct Airside Transit Visa, including U.S. permanent residents with more recent green cards, issued on or after April 21, 1998. The exemptions also include persons with valid U.S. or Canadian visas, who hold tickets to the U.S. or Canada. Diplomats from India and China are also exempt.
It is quite common for the airline agent to incorrectly insist on a transit visa to the U.K. when one is not required, for example, when a person has a valid (un-expired) visa to enter the U.S. It is helpful to print out this information or information directly from the British Government WebSite prior to traveling through the U.K. Rather than being unable board one's flight, such information can be used to clarify procedures on British immigration procedures for the agent.
