If you are passing through the UK in transit, while on your way to a different country, you can apply for this visa.
Make sure you apply for an appropriate sub-category while applying 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 remains 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.