Transit VISA for US from India via London [duplicate]

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.

usa uk indian-citizens transit-visas

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Transit VISA for US from India via London [duplicate] - Public railway station with clock in center

Do I need transit visa if I travel via London to India?

Indian nationals, unless they come under one of the exemptions listed on the UK government web site, need a Direct Airside Transit Visa if they transit airside or a Visitor in Transit Visa if they transit landside - so either way a visa would be required.

Is transit visa required for US for Indian passport holders?

Eligibility for US Transit VisaYou must apply for a visa from India. You must be only passing through the US on your way to your final destination (a third country). You must have no intentions of taking part in tourism or business. For this, you will require a B1/B2 visa.

Do I need a visa to transit through USA from UK?

If you're transiting through USAThose transiting through the USA will need to get a visa or an Electronic System for Travel Authorisation (ESTA) visa waiver. You should consult the US State Department website to determine which you will need.

Do I need a transit visa for a connecting flight in USA?

ESTA is required in all cases of entering the U.S. Travelers must apply for and receive ESTA even when transiting in the U.S. to a third country without a visa. You must apply for ESTA in any of the following cases: transit, transfer, or stopover (layover).

Do you really need Transit Visa for UK

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