The interesting discourse here is why graduates find the big IBs/Consultancies so appealing, more so that joining a startup. In my opinion, this is due to the perception that a high flying blue-chip career gives - 99.9% of graduates would prefer to say ‘I work for Goldmans’ rather than ‘I’m going to work for an unknown technology startup around Shoreditch’ - this impresses grandma and the parents who have subbed the £30-40 university costs for their kids.
I think the counter to this is that within the UK, there is a dearth of success stories involving young tech entrepreneurs - the big one that sticks in my mind is the million-dollar-pixel guy, who as the name suggests made a million! (There are of course others, but I've not seen them as widely covered in the media)
Looking at the example of silicon valley, there are so many examples of tech entrepreneurs making it big at a young age - maybe once the London scene has a healthier number of well known (i.e. the Daily Mail writes about it) exits, the perception will improve and we will see a higher number of bright graduates preferring to join a startup.