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Your parents are travelling on BA with a 7 hour layover in London Heathrow. If they do not have to collect and re-check their luggage which also means that they do not have to go through Border Control (Immigration) then they do not need a Transit Visa.
From the official UK Government website (www.gov.uk) running a quick check states that for an Indian national in certain categories you may be exempt from any Transit Visa requirement.
The stated exemptions:
You donβt need a visa if you have one of the following:
All visas and residence permits must be valid.