I found this information:
https://www.gov.uk/check-uk-visa/y/i...where_else/yes
So basically you need a transit Visa.
But there is this information on that page that stated you might not need a transit Visa. Read below:
Transiting without a visa
You might be eligible for ‘transit without visa’ 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 (marked ‘BC’ or ‘BC BIVS’ in the ‘Remarks’ section) 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)
- You won’t be able to transit without a visa if a Border Force officer decides
you don’t qualify under the immigration rules. You can apply for a transit visa before you travel if you’re unsure whether you qualify for transiting without a visa.