Passing Through the UK in Transit
You can apply under this category if passing through the UK in transit (i.e. on your way to another country).
Make sure you apply for an appropriate sub-category while apply under this category.
Stay within the UK Airport:
If you are changing your flights without going through the Border control (like within the airport) then you need Direct Airside Transit visa (DATV) and it remain valid for up to 24 hours.
Leaving the Airport:
You need a Visitor in Transit visa if you’ll go through the Border control but leaving the UK within 48 hours for your main destination.