The runway at Coventry was not lengthened without planning permission this is an often quoted but not entirely accurate statement. The overrun area at the 05 end was hardened by removing the grass that lay on top of the concrete and putting tarmac down so its to all intents and purposes the same surface as the runway. However unlike the 23 end this has never been given the category of starter strip, hence the difference in TORA in opposite directions.
Your version is not entirely accurate either. If you take a look at the southern end of the runway on say Google Earth you will see the turning circle and the (now mostly erased) white paint arrow markings for a starter strip. This was clearly not built as an overun area. The evidence is still there to be seen. It was only after being threatened with enforcement action from warwick District council, that the 'D' shapes at the edges were covered, and the markings changed to overun area markings