UK Transit Visa
A UK transit visa is a visa that allows you to pass through the UK during transit. There are two types of transit visa. These are: Direct Airside Transit Visa (DATV) which is limits you to 24 hours in the UK - during which you must remain 'airside' (you cannot pass through immigration) - and a Visitor in Transit Visa - which is valid for 48 hours and allows you to change terminals or airports.
If you need to pass through UK you may need a transit visa. To apply for a transit visa you must be:
In transit to another country with enough funds and the intention to travel to your destination;
Able to prove you can enter the country you are travelling to;
Able to show that the only purpose of your visit to the UK is for transit.
Leaving the Airport
You will need to apply for a Visitor in Transit visa if you’ll be going through UK border control but leaving the UK within 48 hours.
You can also apply for a long-term Visitor in Transit visa if you are able prove you need to frequently pass through the UK over a longer period.