The very centre is too expensive - the reason Shoreditch is a centre of startups and artists and the like is that twenty years ago it was where all the crime was. Now the creeping gentrification is happening (as it does everywhere) and some of the startups are moving out to Hoxton or further. Fortunately (and in contrast to SF on its peninsula) there is land all around, so when an area gets too expensive it just pushes people slightly further out.
Shoreditch and Hoxton lost the battle to gentrification a long time ago.
None of the creatives who reinvigorated the area can afford to live there anymore.
Also, London does not have uncontrolled urban sprawl. It is constrained by the Green Belt, which was designed precisely to prevent sprawl. This is one of many reasons property prices are naturally high.
