From Inktober posts Oct 2018 part of the Pick and Choose History of Courtenay Street, Kennington and Lambeth:

Secret Tunnels in Kennington! We all know about the ones from MI6 to MI5 and so on but these ones mentioned by Clowes in 1916 (which he dismisses) might still be there – who knows. When I plotted them on a map they seemed to form a straight line down Kennington Lane to the river – so it might be that the ‘tunnel’ was in fact a dry culvert or old sewer – or perhaps more interesting? There were convictions for smuggling in Lambeth in 16th/17th Centuries and it was also a place where various discarded Queens and pretenders got put (Caroline of Brunswick, Catherine of Aragon and Arabella Stuart) so perhaps also the tunnels were for escape and intrigue… speculations such as this are why I issued myself with an Artistic Licence.