London Heathrow airport - do I need a transit visa? [duplicate]

Per the page you link to, you are eligible for transit without visa (TWOV):

To be considered under this concession you must be transiting and meet the following conditions:

you must arrive and depart by air;

your onward flight must be confirmed, and must depart within 24 hours; and

you must have proper documentation for your destination, including a visa if required

As an Indian going to India, holding a valid US visa and transiting by air, you meet all the conditions. If your flights were more than 24 hours, you would have to either apply for a full transit visa, or change to flights that are under 24 hours apart so you can use TWOV.

I'm assuming from what you've said that you're on your way to the US? And so your full status is an Indian passport holder, Indian resident, with a US visa en-route to the US?

If so, it looks like you should be covered by the 'Transit without visa' concession (scroll to near the bottom of the page and click to expand). The landside Transit without visa concession indicates that as your transit is under 24 hours, you should be allowed to exit the airside during your layover.

The key bit here is that you have a US visa, you're en-route to the US, and your stopover is under 24 hours. If any of those weren't the case, you'd almost certainly need a visa. For the general case, the UK Border Agency "Do I Need a Visa" website wizardy thingy can help you work it out.

I don't think there are many scenarios in which you can get a visa on arrival in the UK. Either you need a visa and you need to apply in advance, or you qualify for some exemption/concession and you are just exempted from the visa requirement without needing to pay a fee or to follow any particular procedure.

If for some reason the UK border force denies you the right to go landside but you are already in the airport under some visa exemption rule then I don't think there is any timely way to get a visa and I assume you have to spend the night there. It's probably the same thing if you are denied entry despite having a visa but I would expect this to be extremely uncommon.

Furthermore, the page on airside transit also mentions a 24 hours requirement for the DATV exemption (presumably to avoid having people stuck in the airport for too long), so it does indeed seem you will need a visa in any case. You will need to get it in advance to avoid being denied boarding in Chicago.

Finally, I would assume that the 48 hours requirement is also a sharp one, which would mean that your other flight option would not only require a “visitor in transit visa” but a (more expensive) short-term visit visa, no matter whether you want to stay airside or go landside.

Info on application process and fees when applying in the US (also note the “Apply for a visa online” in the top right corner).