Medieval Influences and England role in Prog Rock

What we did see is a romantic reimagining of bygone days, (for example psychedelic fashion drew upon the clothes and styling of Edwardian and Victorian eras and mixed them up with Fin de Siècle, Art Nouveau, and Pop-Art), so what you get is kind of distorted pastiche of what someone thinks the music of olden times ought to sound like and not what it did sound like (a bit like Kanye West thinking he is a "rock star") - the stereotypical medieval music we tend to associate with that time owes more to Hollywood than it does to Blondel. Note here that Prog folk band The Amazing Blondel, named after the 12th century minstrel, actually played a reimagined/reinvented form of Renaissance music (ie sounds like Renaissance music but actually isn't).

One way of looking at "why England?" (and not to be too parochial here - this is one of the rare occasions where I do believe it is English rather than British - English musicians absorbed Scottish, Irish and Welsh traditional music as their own) is not that English musicians took American Rock'n'Roll and added other forms of music to it, but like the Italians and Germans did with their takes on "Prog Rock", they adapted it to fit the "local" music they were familiar with. For example trad English Folk wasn't added to Rock'n'Roll and Rock bands didn't "go folk", it was Folk that rocked up and Folk bands went "electric". However, the Folk revival of the 1960s (on both sides of the pond) came after Rock and Roll and were contemporary with Beat music (aka British Invasion) - English popular music traditions before Rock and Roll was Skiffle (musically a kind of hybrid between Jazz and Folk) and Music Hall (not to be confused with Vaudeville). When a Liverpool Skiffle band changed their name from The Quarrymen to The Beatals and became a Rock and Roll band they kept their English roots and adapted Rock and Roll to fit what they knew and were familiar with, having grown up with music hall, church hymns and brass band music. So when they finally settled on the name The Beatles and started writing music of their own they drew upon those musical influences and "rocked it up", and when they were introduced to music that was unfamiliar to them (Classical, Indian and Avant Garde to name but three) incorporating it into their music was a natural thing to do. 

Looking at it that way, it's not so much that English musicians broke away from the verse-chorus song-writing structure and the "traditional" 12-bar and 16-bar blues format of Rock and Roll, but held-on to their more traditional roots when making rock music.