That's not what L4 Means
"In practice a level 4 vehicle is capable of handling "most" road scenarios it may encounter, though retains the capability to throw control back at the human if it can't cope."
Not true. L4 means that the car is capable of full autonomy (i.e. it NEVER needs to hand back control to a human) but only when driving within a particular constrained situation or scenario. In this case that limitation is geographic: they can only drive around one area of Greenwich presumably on roads that have been mapped for the project.