I fish the Derbyshire Wye as well as the River Hodder here. I think one thing to point out (which is made briefly in the video), is that in the UK not all upwinged flies are termed Mayfiles. A mayfly is a separate species and the mayfly hatches happen usually for a very short period at the end of May beginning of June.

Other upwings such as the large olives, blue winged olives etc etc. hatch in spasms at other times. Evening time normally being best but flies can hatch at other times.

I believe that in the US mayflies are any of the Ephemeroptera family of flies.