If TFL really are using the phrase "London underground" they really
area losing the plot...

"London Underground", "London Underground Limited" and even "London's
Underground" could all in some way refer to the network developed from
1863, though of course the use of the title "UndergrounD" to embrace
what were then still independant or associated Tube and Sub-surface
railways came in the early-20th Century. What "London underground" is
heaven knows, it sounds a like part of a terrorist organisation in
itself.

As an aside, for some reason the initials "LU" are now the preferred
choice, even though London Undreground Limited remains a legal body,
albiet under TfL instad of LRT. I always think of LU as a south-west
London bus company, but to confuse matters still further, the Mayor has
organisised several anti-terrorist/anti-racist events under the banner
"London United" - complete with New Johnston script!