Said to have been invented by the cooks at Simpson's-in-the-Strand to either imitate or ward off the 'Pea Souper' Fog, the scourge of Victorian London, when coal smoke combined with the damp air to make a thick yellow smog.
The term is first recorded in Dickens 1852 'Bleak House'; "This is a 'London particular'. I had never heard of such a thing. 'A fog, miss', said the young gentleman." Thereafter, the term 'London Particular Pea-Soup Fog' becomes commonplace. We have not been able to identify when the term was transferred to an actual pea soup, but it appears to be very modern.
The term 'London Particular' - meaning 'speciality of London' - has also been applied to a number of drinks including brands of stout and Madeira.

2016