честно говоря я нашёл другую информацию
http://www.ukvisas.gov.uk/en/doineedvisa/Do I need a UK visa?
If you will be staying in the UK for up to 48 hours before you continue your journey, you will normally need a 'visitor in transit visa'.
However, an Immigration Officer may decide to let you pass through the UK on your way to another country without holding a visa. This is known as 'transit without visa' (TWOV).
И подробности тут
http://www.ukvisas.gov.uk/en/howtoapply ... transit#Q6Do I qualify for transit without visa (TWOV)?
If you are a visa national or a DATV national who is exempt, you can transit without visa if you will:
* arrive at a UK port on a ship and leave on the same ship within 24 hours; or
* arrive and leave by air within 24 hours and have no intention of staying in the UK (you can travel by rail or road between two airports); and
* have a confirmed onward booking that will leave within 24 hours of your arrival in the UK.
Россия попадает в категорию "visa national"
Соответственно если приплыть на корабле на срок не более 24 часов, то должны пустить без визы.
Надеюсь я прав