I've transited through London, and have not needed a transit visa. I am Indian, but I travel on an RTD.
I have traveled thru London many times and didn't need a transit visa when I was a green card holder. But you will need a transit visa if you commute between airports like Heathrow and Gatwick to change flights.
http://boards.immigrationportal.com/showthread.php?t=125674
I found the following info in Britainusa.com for the requirement of a Transit visa:
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You asked if a national of INDIA would require a visa to Transit the United Kingdom
YES - based upon the information provided, a visa WILL be required in this case unless you qualify under one the exemptions listed below.
Exemptions :
1. Holders of Valid U.S. 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.
2. Holders of USA I-551 permanent Resident Cards (Green Cards) issued on or after 21 April 1998
3. Holders of Canadian Permanent Resident Cards issued on or after 28 June 2002
4. Holders of Valid Category D visas issued by EU/EEA member states, which are valid for more than 3 months
5. Holders of Commonwealth format EU Residence permits issued under Council Regulation (EC) No 1030/2002
6. Holders of diplomatic or official passports issued by the Peoples Republic of China
7. Holders of diplomatic of official passports issued by the Government of India
The preceeding categories of traveler are not required to hold a Direct airside transit visa and may still benefit from the Transit Without Visa Concession.
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Also in another page it mentions the same in a more specific language -
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US or Canadian visa
As long as the US visa in your passport is not expired you are
exempt from the Direct Airside Transit visa requirement.
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