So, does one need a transit visa to transit via UK to the US. This question can never be answered in a single word with a Yes or No. There are a few parameters that one needs to take into consideration. I will try my best to put things as clearly and simply as possible.
Situation No. 1 - One doesn't need a transit visa if he/ she is transiting Airside. Meaning, you take a British Airways flight from anywhere in India and stop for a couple of hours for transit at Heathrow to go to take a connection to the US. In this situation, you just need to ensure two things - 1. that your connection is from the same airport and not from a different one (you cannot land in Heathrow and then take a connection from Gadwick or Luton) 2. to get a boarding pass for your connecting flight from Heathrow from wherever you board in India itself (which means by default your luggage also is going to be through checked in). You will not have any issues with this. So if you are getting a low fare on British Airways, do not listen to your travel agent, do not listen to your neighbor, do not listen to anyone, just close your eyes and book your tickets.
Situation No. 2 - One doesn't need a transit visa if he/ she is stopping or breaking a journey to US at any other European countries after transiting via UK. For instance, say you are holding a confirmed ticket on a multiple destination journey like Mumbai - Paris - New York, where you are transiting via London and breaking your journey in Paris (say for a week) and then taking a flight to New York, you definitely do not need a transit visa. In this case, your journey will be broken and you obviously would want to collect your baggage at Paris and re-checkin to go to the US on another date. But still, you will not need a transit Visa since your final destination will be considered as New York (US) as per your confirmed ticked. Only thing you need to ensure here is that you don't change airports and that you have yourself through checked in for your european destination. You will obviously have to re-check yourself in from your european destination to US.
Situation No. 3 - This is the trickiest and the most complicated one. This is further to my recent experience where I had to transit via London to go to the US. But the difference was that I had an overnight halt in London. My flight from Mumbai, a BA flight landed in London at 6.45 PM and I had a connection to New York, the next day at 7.50 AM. So it was a 12 hrs + layover in London. My bags were through checked in and I had a boarding pass too issued for my connecting flight from London to NY. Now, once I landed in London is when I knew that I had I landed myself in trouble. Here, one needs to understand the two terms (one of which I had used earlier) Landside and Airside transits. When someone says Airside, it means that you are not going to get out of the airport. You might land in Terminal 3 in Heathrow and take a connecting flight from Terminal 5 but, you are still considered Airside as you do not have to go through Border Control or Immigration. The moment you land you just have to follow the signs and get to your terminal for your connecting flight. You only have to go through another round of security checks but thats ok. Now, the second term Landside need to be understood very well. It means that you are going to get out of the terminal building/ airport to transit. Landside doesn't really have to mean that you have an over night stay or layover. Even if you are landing in Heathrow and taking a connecting flight from say Gadwick, you need to consider that as an Landside transit. And the rule says that you need a transit visa for the same. Back to my story, I went in to Border Control and showed my passport to the immigration officer. He first asked me about my London transit visa and I said that I do not have it since I thought it wasn't essential as I had a valid US visa and traveling to the US. He refused to listen and told me that I might have to go back to Mumbai. Now, I had one last thing to convince him. I showed my boarding pass and said that my baggage is through checked in. He looked at me for a second and asked me to wait for him while he checks with his senior official. I waited for him as he went to his senior official and explained my case. The senior official to my luck was an Indian. He looked at me from far and I smiled. The immigration officer came back and told me that it was my lucky day, stamped a transit for 24 hrs and let me in. And I happily walked as the doors opened to let me into London. Yes, it was quite dramatic but the story doesn't end there. On my way back, at New York, while checking in, the BA officials made me wait for about 20 minutes looking through the VFS and BA websites to understand whether I can actually travel as I had the same kind of transit where my flight would land in london at around 11 in the night and then I take my connecting to Mumbai at 11 in the morning. Finally I convinced the BA ticketing officials showing my passport and the transit visa stamped there during my onward journey. They reluctantly gave me a boarding pass but they couldn't through check me in to Mumbai. Meaning I had to collect my bags at Heathrow and check in again the next day. And this time when I arrived at 11 PM in London, to my surprise, I wasn't even asked a question. The immigration officer just stamped my passport for a 24 hr layover and sent me in. So it was terrific stroke of not just good but fantastic luck, twice..! Now, back to the question - do you need a visa in a scenario like this. I would say YES a hundred times. You are most welcome to take a chance if you are adventurous. I was and I am. Even if I hadn't been allowed to get in, I would have dealt with it in my own ways and definitely wouldn't have regretted it. But, if you ask me if it is worth it, I would say NO, again a hundred times. Especially, if you are traveling with family or on a tight business sched, i would say just forget it. Because, you are going to spend an additional money on your stay and transport in London. Price vary from 75 Pounds to 300 depending upon the distance from Heathrow. But, one good thing I learnt about breaking my journey in UK is that, you will never have a Jet lag. You are traveling during day time and sleeping comfortably during the night and again traveling the next day.
So, I hope i have tried and explained this to the best of my ability. Have a happy journey:)