Hmmm, according to Wackypedia Imperial is one of the most selective universities in the world with the average entrance statistic being consistantly under 20% i.e. Less than 1 in 5 applicants get in. I know for Chem Eng the statistic is about 18% if you look at Imperials datasheet. This, I believe is even LOWER than Oxbridge.
Also Oxbridge is fantastic, I myself am still devastated over my Oxford rejection, but this "Chasm" that is described between Oxbridge and the likes of Imperial is unfounded. Imperial is rated 1st in Europe for science based courses. In fact there have been instances where Imperial is above Oxford in the league tables.
So to answer the opening question...Yes, its bloody hard to get into Imperial.