I noticed number of visits for one of my previous post about the stopover at London Heathrow airport. This happened in 2007 and me and my wife both were in transit to USA after our vacation at Sri Lanka. We had a 15 hour layover (or stopover) at Heathrow and we were able to obtain 24 hour entry visa to UK and spent few hours at my cousins place before catching the flight next day.
See my previous post here.
I'm not sure how the law regarding this 24 hour visa, and I'm still getting number of emails requesting more information about this. So, I did a quick search and found following information, so this will help you deciding whether you need a transit visa before the travel or whether you can obtain visa on arrival through the UK Border Agency. But most of the time it is a safe bet just to contact your airline to make sure that you need or need not transit visa.
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This questionnaire will help you to decide some of it.
Do you need a visa?
This section explains how you can transit through the UK when traveling to another country. It also explains the documents that you will need to bring with you.
If you pass through the UK without staying here on the way to another country, this is called 'transiting'.
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There are two types of transit through the UK:
You will transit airside if you arrive on a flight, remain in the arrival lounge of the airport without passing through immigration control, and then depart on another flight from the same airport.
You will transit landside if you arrive on a flight, pass through immigration control and then leave the UK within 48 hours (or 24 hours if you are travelling under the 'travel without visa' concession).
If you want to spend longer than 48 hours in the UK before continuing your journey, you must come to the UK as a visitor.