Some Visa nationals, provided they arrive by air and have a confirmed onward ticket for a flight within twenty-four hours to a country where they are assured entry, do not require a transit visa and can leave the airport. However, being able to transit without a visa is only a concession and there is no guarantee that someone will be allowed to pass through the UK in transit by the UK Immigration Authorities. You may still be refused entry when you arrive at a UK airport if a UK Immigration Officer is not satisfied that you qualify. You may wish, therefore, to actually apply for a UK transit visa before you travel.