Their ship was buried underground, and they were trapped there, stuck in their clockwork bodies. The Enochai had previously tried to manipulate John Dee, but he stopped them, destroying some of their physical bodies and trapping one of them in cryogenic suspension. The Enochai who had lost their bodies possessed the trees in Greenwich.
The Enochai were a race from another universe made of pure energy. As such, they required a vessel to inhabit. Normally, they lived in winged humanoid clockwork bodies. However, if their clockwork body was destroyed, an Enochai could possess another form such as the trees of Greenwich Park.
Individual Enochai were capable of flight and could fire deadly blasts of energy from their hands that could easily vapourise their targets. They could also form an energy link between each other to create a dome of energy that sealed off all of London from the rest of the world.
The Enochai that possessed the trees of Greenwich Park could animate them as they pleased. They were strong enough to effortlessly crush humans and were highly resistant to damage from flamethrowers or missiles. (COMIC: Don't Step on the Grass)