The origins of progressive rock (proto-prog)

Many British musicians would strongly argue that UK Prog began with Billy Ritchie and his band "1-2-3".  Their residency in late '66 and early '67 at the Marquee introduced musicians as diverse as Jon Anderson,  Jeff Lynne, Deep Purple, David Bowie and Keith Emerson to the idea of complex orchestrations played in extended works.

Unfortunately, there is little by way of recordings of this band - there are a couple of poor quality tape-recorded bootlegs - but they never really got into the studio in their original form.

In 1969, a change of management lead to a change of name.  They became "Clouds" and released two albums under that name.  Sadly, according to people (mostly musicians) who'd taken the time to seek them out in their original incarnation, "Clouds" was a shadow of the former band.  They allowed their management to push them into performing "more accessible", lighter "pop" material.

Billy Ritchie gave up the music business in disgust in 1972 or so.   Progressive music lost one of its greatest innovators.

It's worth reading "The ABC of 1-2-3 by Billy Ritchie himself, and  "The Story Of Clouds" by James P. Alexander.