It's a fact that's often repeated on various websites and books, but not really true. Admittedly, he installed his toilets at the Great Exhibition, but the Exhibition's commissioners had been lobbied to take action by a committee of members of the Royal Society of Arts.
It's often said that he installed underground toilets at the Royal Exchange in 1855 but this claim seems to date back to 'The Good Loo Guide' - a comic 1960s publication by Jonathan Routh - which gets the date wrong by some thirty years - 1885 is the correct date (click here for details), and Jennings was not involved in that particular build (not least because he was dead three years earlier), nor his firm.
He did, however, offer at least twice to set up public toilets in the City of London, and came up with the idea of underground public toilets in 1858, which were rejected due to 'English delicacy of feeling' (i.e. public water closets were considered unsavoury and immodest). [I only have an account of this response from Lawrence Wright's 'Clean and Decent' 1960, p.201, which reprints a memoir/letter/comment from Jennings without providing a citation]. Jenning's company, which survived his demise and prospered, would provide fixtures and fittings for many of the public toilets built in the 1880s and beyond, and contracted to manage some of them, in prime locations in London.
I've been mooching around the archives at the LMA (which I love, but the City's sanitary records are the worst-catalogued collection of material in the known universe) and, although I haven't found the City of London's rejection of Jenning's proposal, I have found his original 1858 letter. Here it is, in full:
5 Holland Street, Blackfriars, S.E.
December 13th 1858
To the Honourable the Commissioners of Sewers for the City of London
Gentlemen, I observe in 'The Times' that certain matters of a Sanitary character have been referred to your Engineer.
I think it only right to call attention to the efforts I have made to prevent the defilement of our thoroughfares and to remove those Plague spots that are offensive to the eye, and a reproach to the Metropolis.
Having provided, and fixed the Sanitary appliances at the Exhibition of 1851 and the Dublin Exhibition of 1852, as also those in the present Crystal Palace, I can bear testimony to the Public application of conveniences, suited to the advanced stage of civilization.
I know the subject is a peculiar one, and very difficult to handle, but no false delicacy ought to prevent immediate attention being given to matters effecting the health, and comfort, of the thousands who daily throng the thoroughfares of your City.
Herewith, I send you Lithographed drawings of a 'Halting Place' for a public Crossing: the space proposed to be occupied is not more than is appropriated to the any of the present 'Halting Places' and the design also provides for a Public Lamp & Drinking Fountain.
In the super-structure (which may be extended to any size without in the slightest degree impairing the stability of the thoroughfare above) you will see I show a Public Urinal, similar to those I have so satisfactorily erected for the St. Giles District Board of Works in the Seven Dials, and at the British Museum, as alson on the Great Northern and other Railways.
Water Closets and Lavatories are also shown with a Room for the Attendants. The wall space I propose should be covered with Time Tables from the different Railways; lists of Cab-fares from that particular point, to any part of the Metropolis, and other useful information. You will also observe that every arrangement is made for lighting and perfect ventilation.
In 1851 in connection with the Society of Arts, I spent a considerable sum to promote the establishment of Public Waiting Rooms and in the beginning of 1856, I had elaborate drawings made, showing how the vacant space next to the Royal Exchange might be advantageously employed. These drawings were submitted to the then Gresham Committee.
Public Waiting Rooms - Fountains - Water Closets and Lavatories may by some be considered a little in advance of the times, but they are nevertheless a necessary and would be appreciated accordingly.
As an example - At the Crystal Palace,for want of funds, the Lavatories were nearly excluded. I urged their introduction, but was told "persons would not come to Sydenham to wash their hands." I persisted and ultimately at a nominal cost, fitted the whole of the Waiting Rooms. These Waiting rooms I am told now produce a thousand pounds per annum.
If you consider my arrangements bear the stamp of common sense, I offer through you to supply and fix in any part of the City every Sanitary appliance shown in my drawings free of any charge.
If the cost of Gas-lights Water Supply and respectable Attendants capable of understanding and answering a question, should be considered an expense too great to be encountered, I am willing to take every expense on myself, and carry out every detail, under your Engineer's direction, provided the attendants I furnish are allowed to receive a small gratuity for use of Towels &c. as at the Crystal Palace.
If this (which many would turn a speculation) should prove commercially valuable, I shall be quite ready at any time to resign my trust, or retain it, using the proceeds for the establishment of similar conveniences.
I have the honour to be My Lord & Gentlemen,
Your obedient Servant,
Lithographed Drawings of the proposed 'Halting Place' were laid before the Court, and examined.
That it be referred to the Committee on General Purposes to consider the said proposal and report.