You'll need a visa to pass through the UK in transit (unless you're exempt)
You should apply for aVisitor in Transit Visaif you arrive on a flight and will pass through immigration control before you leave the UK. Transiting without a visa:- You might be eligible for the 'transit without visa concession' if:-
you arrive and depart by air,
have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive,
have the right documents for your destination (eg:- a visa for that country),
One of the following must also apply:-
you're travelling to (or on part of a reasonable journey to) Australia, Canada, New Zealand or the USA and have a valid visa for that country,
you're travelling from (or on part of a reasonable journey from) Australia, Canada, New Zealand or the USA and have a valid visa for that country,
you're travelling from (or on part of a reasonable journey from) Australia, Canada, New Zealand or the USA and it's less than 6 months since you last entered that country with a valid entry visa,
you have 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,
you have a residence permit issued by Canada issued after 28 June 2002,
you have a uniform format category D visa for entry to a country in the EEA or Switzerland,
you have an Irish biometric visa and an onward flight ticket to the Republic of Ireland
you're travelling from the Republic of Ireland and it's less than 3 months since you were last given permission, on the basis of holding a valid Irish biometric visa, to land or be in Ireland,
you have a valid USA permanent residence card issued by the USA on or after 21 April 1998,
you have a valid USA I-551 Temporary Immigrant visa issued by the USA (a wet-ink stamp version will not be accepted),
you have 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,
you have a valid standalone US Immigration Form 155A/155B issued by the USA (attached to a sealed brown envelope),