Thanks for the info, I am from India.
The information on the http://www.ukba.homeoffice.gov.uk/visas-immigration/visiting/transit/visa/ is slightly ambiguous, hence can somebody from India who has traveled via Heathrow/London please confirm whether
The nationals of some countries are known as visa nationals, because they normally require a visa to come to the UK. These countries are listed under 'More information' below. Nationals of other countries are known as non-visa nationals.
You do not need a visa as a visitor in transit if:
you are a non-visa national; or
you hold a valid travel document issued by the UK government; or
you meet the 'transit without visa concession' - see 'More information' below.