I think here it's worth distinguishing between actually ecologically important, and the ability of campaigners to raise environmental arguments to try to prevent development when their underlying reason is something else (i.e. not wanting a nuclear plant in their back yard). There are definitely ecologically unimportant places in England, but it's almost certain that if you try to put a power station there somebody will raise an environmental concern anyway. This will probably eventually be dismissed, but you can gum up the works procedurally for a pretty long time.

City of London? Does it have much ecology left? Then again I suppose too many people would raise a fuss...

Slough

Birmingham?

Salford

Anywhere in London would be good. There's nothing else worthwhile there.