You might need a visa to pass through the UK in transit (on your way to another country). 

You can use this tool to check if you need a transit visa You can find additional information about Transit visas here.

You will need the following information to do the check:

• The nationality on your passport 
• Reason for travelling to the UK (Transit, on your way to somewhere else)
• Where you ae travelling to, there are different rules for the Republic of Ireland, the Channel Islands and the Isle of Man.
• Will you pass through UK border control? You always pass through border control if you leave the main airport building for any reason or if you need to collect your bags and check them in for your onward flight. You must also pass through border control if both your onward flight leaves on a different calendar day than when you arrive and there’s nowhere for you to stay overnight in the airport, for example in a transit hotel. 
  Check with your airline if you’re not sure if you’ll pass through border control. There are more than one airport in London (Heathrow – LHR,  Gatwick – LGW and London City – LCY) 
• Your Exemption status. If you are in the EU on a category D visa or have Permanent Residence in the EU you might be exempted from needing a Direct Airside Transit Visa or a Visitor in Transit Visa

Exemptions that may apply if you have one of the following

•  a common format residence permit issued by a European Economic Area (EEA) country or Switzerland
• a uniform format category D visa for entry into a country in the European Economic Area (EEA) or Switzerland