Пересадка в Лондоне

Очень часто спрашивают, что нужно сделать, чтобы выйти в город, во время транзита через Лондон.
На сайте британского визового центра написано следующее (то же самое где-то есть на сайте UK Border Agency):

Do I need a transit visa if I am staying in the United Kingdom for less than 24 hours? Can I leave the airport?
If you are a Russian national, provided you arrive by air and you have a confirmed onward ticket for a flight within 24 hours to a country where you will gain entry, you do not require a transit visa and you can leave the airport. Being able to transit without a visa is only a concession and there is no guarantee you 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 the UK airport if a UK immigration officer is not satisfied that you qualify. We recommend that you apply for a UK transit visa before you travel.