I had this exact problem driving up the steapest col in the Pyrenees in France recently. It was the rear right disk while driving on the right, so assumed it was wayside grit - but it went on for 30 miles. The passing looks of 'thought electric was supposed to be quiet' told it all. I did immediately stop and tried to clear it by back and forth shuffling, then by waggling the (very flimsy!) plate - all to no avail.
As I still got the odd scraping sound after that, I was worried that perhaps a locating screw or similar had got stuck against the plate, but after driving 800 miles home a few days later without mishap, and with its 1st service due a few weeks later, I took it up with the engineer - who reassured me that all was well. I had also mentioned that the brakes had become very sharp at very low speeds, and he did make the very sensible suggestion (they seem to be one of the 'good guy' authorised MG service only garages) that the brakes were very little worn and I evidently used the KERS a lot (correct!) so he advised every so often turning regen down and using the brakes HARD for a while to clean them up a bit.