You are exempt, even if you need to clear immigration, you will get a 24hr visa free of cost. Max. transit time is 24 hours.
Indian nationals are exempt under following situations:
- a visa which allows entry into (or transit through) either Australia, Canada, New Zealand or the USA; and
- holding a valid ticket for travel from a third country or territory via the UK as part of a journey TO (or through) Australia, Canada, New Zealand or the USA, whichever the visa is applicable to (e.g.: DEL-LHR-JFK, DEL-CDG-LHR-JFK, DEL-LHR-CDG-JFK or DEL-LHR-JFK-MEX).
OR:
- a valid visa for entry into Australia, Canada, New Zealand or the USA; and
- a valid airline ticket for travel via the UK as part of a journey FROM the country in respect of which the visa is held to another country or territory.
OR:
travelling as part of a journey FROM either Australia, Canada, New Zealand or the USA, provided transiting the UK less than 6 months after the date they last entered Australia, Canada, New Zealand or the USA with a valid visa for the respective country, even though the visa may have expired at time of transit through the UK (e.g.: JFK-LHR-DEL, JFK-CDG-LHR-DEL or JFK-LHR-CDG-DEL).
NOTE: this DATV exemption is not applicable if holding a US Visa Foil type "YY" or "ZZ" endorsed "Not a visa. Foil prepared at DHS request" nor the US Adit stamp worded "Processed for I-551" (temporary form I-551).
Source:
Personal experience(been there, seen there)