This question already has an answer here:
I know this is a kind of duplicate question but my scenario is bit different and I am looking for the answer as per my scenario.
My parents will be travelling from India to New York via London after 2 weeks, their flight has a layover of 2.5 hours at London - Heathrow. Their next flight will be departed from the same airport i.e. from London - Heathrow after 2.5 hours but before mid-night of same day (as per schedule of flight). They have a valid B1/B2 visa for United States.
Do they need for transit visa at London-Heathrow. I have already gone through link "https://www.gov.uk/check-uk-visa" but some says that Indian passport holders need transit visa. Please let me know if they require it or not.