A few years ago I had a Transit van with an intermittant fault on the speedo. The day I took it for mot it stopped working on the way there. However, the van passed as the speedo is not an item that needs to be checked for mot. Of course I could still get done for speeding.
So, it seems to me, that speed limits can and are applied to vehicles that have no need to have a working speedo.
The issue still is the identification of bikes and riders who are found exceeding said limit. Any plate would be much smaller than a standard vehicle plate. (Are we really going to see the wigins' and Fromes of this world riding around with a full size plate on the back?). So even if the plate is readable by camera or human at distance, who is the rider?
12 year old Johnny gets stopped for speeding. "Is this your bike sonny, can I see your insurance?" Really?
There are so many holes in this whole thing. So many changes that need to be put into place, I can't see it happening. If only 'cause it will kill cycling at the very time it is starting to grow as a utility activity.