The fact is for undergraduate degrees, this departmental ranking stuff is mostly rubbish. Let me explain why.
For instance Lancaster's Physics department is extremely highly ranked, possibly the highest, and yet their admissions requirements and graduate prospects are lower than equivalent courses at Oxford, Cambridge, the top London universities, etc.
Why is this? Well it is because prestige matters. People like to think their Undergraduate degrees are oh so difficult (I am an undergrad myself), but the truth is that it is still very much school-like in what you learn. Nothing is truly cutting edge, or academically that hard, and could mostly be taught from a bunch of good text books. If this is the case, then what makes one department "better" than another?
Research? For graduate programmes, sure, but for undergrad? I don't think so...
The thing which puts Oxbridge head-and-shoulders above the rest is the tutorial system. People can bring up whatever rankings they like, but the truth is that I have covered a lot more material, in less time, in Oxford than the better ranked course in Imperial College.
Secondly, prestige. If I wanted to be the absolute best Computer Scientist, perhaps I would have picked Soton, however that isn't what I want to be. Realistically they are so close anyway, things like atmosphere and prestige tip the balance. Oxford teaches 'soft skills' which are valued in the industry I want to go in to (finance), and the international prestige is important, Southampton simply isn't targeted in the same way Oxford is.
If you want to be an academic, nobody cares that much where you got your degree. If you did well, and convince them you can do the research, it doesn't matter where you got your degree. So the real question, what does a "higher ranked department" offer that is beneficial?