No lecture courses are taught by PhD students at Imperial and I doubt they are at any other good university either. PhD students can take tutorials, and mark coursework (not exams) however.
You are right that being a good researcher doesnt necessarily make someone a good lecturer though, and since the top universities mainly look at research when hiring staff, its possible to get poor lecturers. That said, a good researcher is more iikely to have a deep understanding of the subject, and hence has more insight into which parts are really important, which can make the courses they teach to undergraduates more interesting