Not sure why Falmouth is being brought up on this thread - it's a good example of a line that has been invested in, seen a frequency increase to catch up with increasing passenger numbers.
It's short, simple, self-contained, it works without any grandiose long distance aspirations - each train only does around twenty miles each hour but I don't think you could squeeze it down to provide a half hourly service with just one unit (doing forty miles in an hour, given the intermediate stops etc - unless you ran half the journeys as non-stop), so it's probably just as well to leave it as it is.
Which routes are there in England that fit that description? I can't think of very many if any. Scotland and Wales are politically different and so I just can't see any closures there, even the obvious basket cases (Conwy Valley, Heart of Wales, Far North).
The lines that must be most under threat, other than maybe the middle of the Cumbrian Coast, are in Scotland and Wales
Oh, there's plenty of lines in this part of England that could lose passenger services without many people being affected - you could either just give up on irrelevancies like Goole - Knottingley or you could replace lines with irregular frequencies (Pontefract Baghill, Brigg, Skipton - Lancaster, Doncaster - Lincoln) with just one train a day/week.
A few rural stations could be cut back to just a peak time service, even if the line frequency isn't affected, to free up a unit and save a few quid. You could reduce Edale to just a weekend service and still retain 90% of the passengers.
I don't see any lines closing altogether but there are certainly a few stations or lines that I'd be worried about if I lived there.