There's definitely a lot of variety in the London rental market, which isn't surprising given the size and age. I've had central controls, single thermostats, thermostatic radiator valves and, in one particularly sketchy conversion, radiators that were connected to someone else's heating system.
However, I wouldn't have paid someone to install the Nest thermostat in any of these properties because I didn't own them and I wasn't planning to stay long enough to benefit. I can't imagine that it would have been of any benefit to my landlords to install one either.
I presume that Nest's target market is homeowners, but their statement that there aren't many thermostats certainly appears to be based on anecdotal evidence from the rental market.