Orchestrated Rock...Uriah Heep and others

“Tommy” by the Who is a brilliant work of art by one of the great rock bands ever. It is truly a rock opera & has much orchestration. The original recording isn’t bad & the reissue was cleaned up a bit & overall better sounding. Turn off the lights, turn it up load & enjoy!

”Life’s a Long Song” by the brilliant Ian Anderson  of Jethro Tull fame is a beautiful song from their “Living in the Past” in 1971. He has often performed w/ full symphony orchestras many of his classic hits. There’s a great version of him w/ a string quartet, drummer & piano on YouTube.
Try this: YouTube.com/watch?v=nys-gSP8oVE

There's always "In the Court of the Crimson King"

My favorite Uriah Heep record is Demons and Wizards. 

@Jasonbourne - for the avoidance of doubt, I don't have the recording in my collection - which has all the other Purple albums. I was just citing it because it is a "does what it says on the tin" in terms of the question the OP asked.

Similarly, I'm not a fan of ELP's mangulation of the classical catalogue.

In fact, the majority of prog rock hasn't worn well for me.

Renaissance - "Turn Of the Cards"

The Babys - "Broken Heart"