Holders of certain documents are, regardless of nationality, exempt from the requirement to hold a Direct Airside Transit Visa when transiting the UK.
A transit passenger is not required to hold a transit visa if he holds, or a person with whom he arrives in the United Kingdom holds on his behalf:
a valid visa for entry to Australia, Canada, New Zealand or the United States of America and a valid airline ticket for travel via the United Kingdom as part of a journey from another country or territory to the country for which the visa is held;
kaynak:
www.ukvisas.gov.uk