[1][5] Preston returned to London for good in 1777, and entered a business partnership with his friend Charles Foulis, who had previously managed his voyages.

[5] Together with Charles Foulis, Preston established himself as an insurance broker with premises in Old Jewry in the City of London,[2] and both men became managers of the Sun Fire Office.

[4] In 1781 he was made an Elder Brethren of Trinity House, the official authority overseeing lighthouses in England, Wales, the Channel Islands, and Gibraltar, eventually becoming the corporation's Deputy Master from 1795 to 1803.

[7] In the 1784 British general election, Preston won one of two seats in the parliamentary constituency of Dover for the governing Tory party of William Pitt the Younger.

He was friends with politicians William Pitt and Henry Dundas, diarist James Boswell, painters Alexander Nasmyth and J. M. W. Turner, and poet Sir Walter Scott.

[4] When traces of coal were discovered on Preston Island–a cluster of rocks in the Firth of Forth's intertidal zone off the coast of Valleyfield–in 1805, Sir Robert established a colliery there.

[4] In this Preston explicitly followed the example of Sir George Bruce of Carnock, who had created one of the world's first offshore coal mines in nearby Culross in 1617.

Alongside a small stone bridge (today a C-listed building), a number of plain classical houses were built to accommodate retired sea captains.

[14] Nearby Preston Hill between Inverkeithing and Dalgety Bay is also named for Sir Robert, who erected a flagpole there intended to aid marine traffic.