(Taken from the UK Immigration website)
You might be eligible for ‘transit without visa’ if: you arrive and depart by air have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive and "have the correct documents for your destination (eg a visa for that country)
Two passengers, One a US passport holder and one from Thailand with a valid USA Visa, will be arriving from Bangkok by air but are departing Heathrow to the UK Southampton Pier on the same day for a direct transatlantic Cunard Cruise to the USA
Would this situation suffice for a Heathrow to Southampton Pier "Transit without a visa: and allow air travel to Heathrow without a UK visa?