Hmmm...
Unfortunately, since drones have become high profile, they're the first thing that get blamed when pilots see something. Notably UFO sightings have dropped in inverse proportion...
In this case something 2m across, 'balloon like' and at 5,000+ feet is NOT a drone of any sort that you can buy or even build without serious commitment. Any one of those descriptions rules out just about anything available commercially unless you're spending tens of thousands of pounds or work for the military. The largest commercial drones are under half that size, and most would struggle to reach 5,000 due to battery limitations.
So either, it's someone with Bond-villain levels of commitment and finance, or it's not a drone. The most likely suspect is either a hobbyist playing with high altitude baloons, or a stray party prop.
Of course, we could just ban/license/arrest everyone flying a drone - but it has to be pointed out that the commitment to stupidity that would be needed to cause an incident like this would not be stopped by harassing kids playing with toys, or licensed drone operators doing their jobs.
Perhaps a first step would be to put a gimbal mounted high resolution camera on each plane so that unexpected objects can be accurately identified and the threat assessed? Compared with the cost of any of the alternatives, a few hundred quid for a camera would not be so unreasonable.