They start up to make a profit, not tread water until they are bought by a bigger company.
The majority of the inhabitants of "silicon roundabout" are carbon copies of whats been before. whats more the service they provide is usually free. So there is no hope of long term success without a buyout.
In short, the very definition of a startup is unsustainable.
Several major exchanges (in energy, commodities) are recently ex-startups, and there is an ecosystem of innovative, fast growing companies servicing the city. Because finance is what London does.
Outside Central London, often clustered around universities (Cambridge, Surrey, and the Thames Valley) are interesting non-internet start-ups. Chip design startups, making IP they hope will be acquired by ARM or Qualcom. Startups creating software for the UKs huge, NHS motivated big pharma presence. These are B2B companies that you don't hear about unless you are involved, so I'm sure there are plenty popular niche I'm unaware of.
But Silicon Roundabout, or UK web service companies? I've looked at the companies there, hoping for one that looks interesting enough to grow, interesting enough to join. Nothing caught my eye. Datasift maybe.
I guess my comment was a bit negative. I wish good luck to anyone trying to make it. You'll have the last laugh when you cash out.