Portsmouth, contrary to some propaganda otherwise, has seen a reduction in KSIs on 20mph roads and is not choking under increased pollution. In some areas (eg Calderdale) 20mph schemes are being led- and paid for- by public health departments as the benefits are now proven.
(as for enforcement, no fixed cameras are type-approved for 20mph, only average speed cameras and hand-held measurement)
Country roads? Most them are simply unsafe to be driving at 60mph down, unless you're a rally driver and the road is shut for a race. if you want to see what lower limits looks like in practice, Clackmannanshire, the smallest rural mainland highways authority, has more or less done the entire county, with a mix of 30, 40 and 50 as appropriate.
(Yes I have strong opinions on this. it was interesting to see at least week's 20mph conference in Cambridge quite how seriously this is being taken by TfL, TfGM and smaller authorities across the country)