There's an alternative here.
The Citadel may not be in a stationary orbit over London. It may, however, be in a very slow orbit around Earth such that it remains lined up with the beam for a reasonable amount of time. That just means that the assault on London had to be done at a specific time - which doesn't actually contradict anything in-game.
I've just run some trig with a screenshot from space during Priority Earth (just after Hackett receives word that Shepard made it to the Citadel). You can see the UK in the background, you can also see the Citadel, and knowing what size they are compared to what size they appear on screen allows you to calculate how far away they are.
If you trust my math (I do
), and if you're willing to put that much faith in a cutscene (eh...
) the Citadel comes out at an orbital height of tens of thousands of km. Even in an ordinary orbit, it would remain over the UK for a matter or hours that way. Which solves both problems - it won't fall to Earth, but it will remain lined up with the beam for a reasonable amount of time.
There's one remaining problem though - as I mentioned before, the Crucible shockwave is travelling along to the ground in the ending cutscenes, which are set in London. For that to happen, the Citadel can't be above the UK anymore, otherwise the shockwave would have come down from above, rather than from the horizon. In other words, the Citadel must have moved quite a distance between when Shepard uses the beam, and when Shepard activates the Crucible. It has to have reached somewhere that is 'on the horizon' relative to London.
You know, I could probably use that information to make a more accurate guess at the Citadel's orbital velocity... but somehow I doubt it'll line up with the height I figured out earlier, because, well, I seriously doubt the Bioware cinematic designers put this much thought into it.