People might have some degree of infrastructure to build a start up down here (especially around the Old Street area) and there's plenty of office space but the problems lie in the fact that if you want to employ young graduates (or people with a few years of experience) you're going to struggle to pay them without adequate investment.
A 1 bedroom property down here will cost you roughly £1200 a month + bills (£1300 - £1400 in total) and then travel in zones 1 and 2 will cost you £106 a month for a travelcard
If you're going to pay a youngster £30k a year then they're going to be forking out 70-80% of their income on accommodation which is absurd.