League tables are worth looking at, but don't base your decisions solely on them. Just because York is one rank above Warwick does not mean it has the better department. It's just an arbitrary difference affected by subjective factors. Same goes for Manchester.
I agree you should probably not bother with Manchester though, it actually has a world-class computer science department and excellent graduate prospects, but not worth the extra effort when you have less demanding offers from unis of the same calibre.
As far as employability goes, all of your universities will give you a head start. Obviously Imperial is one of the best, if not the best, engineering/science schools out there, but you won't be at a big disadvantage going to Warwick, York, Durham or Manchester instead. As long as you get into a top uni, the rest is more up to your own capabilities.
Now to compare Warwick, York and Durham, you need to focus more on the course content. Generally speaking York has a more hardware-oriented programme while Warwick puts greater emphasis on the maths aspect of computer science. Durham is highly regarded as an alternative to Oxbridge, but I'd say its real strength is with social sciences. This is not to say the computer science department at Durham is weak, but you'd be better off going to a more sciencey uni, like the other four on your list.
In terms of prestige for computer science, it goes as follows (again, with mostly marginal differences):
Imperial
Warwick/York
Manchester
Durham