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 don’t need a visa if you have one of the following:
- A visa for Canada, New Zealand, Australia or the USA (this can be used for travel to any country)
- A residence permit issued by Australia or New Zealand
- You have a common format residence permit issued by an European Economic Area (EEA) country or Switzerland
- A resident permit issued by Canada after 28 June 2002
- A uniform format category D visa for entry into a country in the European Economic Area (EEA) or Switzerland
- An Irish biometric visa
- A Schengen Approved Destination Scheme (ADS) group tourism visa where the holder is travelling to to the Schengen country that issued the visa
- A flight ticket from the Schengen area, if you can prove that you entered the Schengen area in the previous 30 days on the basis of a valid Schengen ADS visa
- A valid USA I-551 Temporary Immigrant visa issued by the USA (a wet-ink stamp version will not be accepted)
- A valid USA permanent residence card issued by the USA on or after 21 April 1998
- An expired USA I-551 Permanent Residence card issued by the USA on or after 21 April 1998, with a valid I-797 letter authorising extension
- A valid standalone US Immigration Form 155A/155B issued by the USA (attached to a sealed brown envelope)
All visas and residence permits must be valid.