A FiT just obscures the problem
No matter how you slice it, a FiT is still just another government-enforced subsidy. By definition, it's not economically efficient nor desirable.
Yes, our power plants are getting old. But they don't need to be replaced with more expensive renewable power when cheaper and abundant energy such as gas and newer generation coal are available. You mentioned that England has high-priced natural gas, but you failed to mention that recently it discovered shale gas in its north that rivals the largest US fields. That will be coming online shortly.
Reaching grid parity means nothing in the overall energy scheme. Sure, it's nice when solar and wind reach "grid parity" -- except that the sun doesn't always shine and the wind doesn't always blow. You still need coal and gas plants to back up renewables, or some form of expensive storage. The problem is that once renewables become more than about 20% of your mix, you wind up having coal and gas plants that sit around idle for much of the time, greatly increasing their capital costs. Lack of compact and cheap storage options for electric also mean that renewables won't allow us to escape our reliance on oil any time soon.
