Truly an excellent thread – well done everyone. Quite inspiring stuff. My thoughts (before seeing other posts) were also:
[*]The Mersey Viaduct in Stockport – you just want to count the bricks – it's staggering![/*]
[*]Winter Hill – anything you can see for so far around is an achievement in itself.[/*]
[*]The tunnels under the likes of Liverpool, Manchester and of course London – weird when you think about it.[/*]
[*]Ladybower reservoir, and everything that goes with it.[/*]
[*]The M62 betwixt Manchester and Leeds – fascinating to think how they built it.[/*]
However – what about these (from where I grew up)
Some seriously old engineering at Maiden Castle (and there's a bridleway along one edge of it, IIRC.)
Head north west on bridleways along the Ridgeway and you get to Hardy's Monument (Admiral, not the author)
Just visible from the monument (if you look South) is Portland Harbour – impressive in itself, but home to remnants of the Mulberry Harbours used around D-Day.
Great stuff – thanks again, OP.