From observing reenactments of early American life, I think an 18th century agricultural technology is self-sustaining and directly bootstrappable, except for the problem of mining and working iron. So my guess is not much longer than 300 years.
The argument is that getting from animal-powered devices to solar/nuclear/whatever powered devices while at the same time switching from 90%-agricultural workforce to anything more progressive can happen only if there is a cheap source of energy available - and we already have mined and spent all of easily available fossil fuels.
Even if all kinds of fancy devices are available and constructed by rich enthusiasts, the lack of cheap steam power ensures lack of cheap steel/etc, and all the technologies don't get the mass adoption required for their improvements, there are almost no advantages for industrialization, so the world gets stuck in feudal-agriculture systems as the local optimum.
http://opinionator.blogs.nytimes.com/2012/03/13/in-africas-v...
It wouldn't have quite the pop of surface-minable coal and oil, but if you were pushing for high technology it would boil water, drive steam engines - but maybe it wouldn't happen in England. Where you'd get the metal from is another question.