This is not some weird oddity due to council mismanagement, it's just the latest example of one of the greatest naturally occurring scams in history.
The process that's occurring is Ricardo's law of rent: https://en.wikipedia.org/wiki/Law_of_Rent#The_Law_of_Rent
It's counter intuitive, but brilliant once you realise how it works. Put simply, when a company makes more money due to the location they're in, the landlord gets all of the extra profit, not the company that does the work. Every time the company's profits rise, the rent will go up by the same amount. Free money without working if you own land, and very bad for capitalism.
In the case of Old Street, it used to be that there was little advantage to being there, so rents were cheap (not much competition). That attracted lots of startup types and now, the area confers a significant advantage due to the network of companies and people. The choice for a new startup is :
a) Old Street, where recruitment costs (for example) will be lower, or
b) Somewhere Rubbish, where you pay e.g. an extra 20k per year for recruitment.
If the rents are the same, everyone will want Old street for the 20k advantage, so the landlords play them off against each other: the offices go to the highest bidder. Whilst the extra rent you pay to be there is less than 20k, it makes sense to pay it as you're still ahead, so people keep upping their bids until the rent reaches ($rent_for_rubbish_place + 20k). The landlord gets it all for doing nothing!
It's a scam because it operates outside of free market capitalism: you can't manufacture more land at Old street roundabout (or move it there from elsewhere), so supply does not increase when the price rises. Not a free market. Also no jobs created by the price rises.
The solution to the scam is land value tax (LVT), which would reclaim the unearned money from the landlords (whilst leaving them their fairly earned money from providing the buildings). LVT done right would make corporation tax unnecessary whilst causing no price rises and no job losses. A massive boost for the economy. Seriously. No hyperbole - it works counter intuitively due to the fact that its not a free market in the first place.
Winston Churchill was strongly in favour of LVT and described land as 'the greatest of all monopolies'. He thought that taxing it was one of the most important political ideas of the century, so it warrants a bit of reading: http://www.landvaluetax.org/current-affairs-comment/winston-...