Direct Airside Transit visa. You might need a Direct Airside Transit Visa (DATV) if you're from outside the European Economic Area (EEA) and Switzerland and you: will be changing flights in the UK on your way to another country.
There are two types of transit visas for the United Kingdom: A Direct Airside Transit Visa (DATV) is valid for 24 hours and does not allow you to pass through the UK Border Control (i.e. you must remain 'airside').
At London (Heathrow), there's the Direct Airside Transit Visa (DATV). “If you're transiting London you need DATV. But if you're traveling, say, to the US, there are exemptions. ... But if you're flying to the West Indies and you need to change airports and go from Heathrow to Gatwick, then need a transit visa