Concorde development was a (UK & France) national project. They would have had easy access to military aircraft. Aircraft like the Lightning might only just have been able to intercept but would easily have observed pre-arranged tests.
It's almost inconceivable that the test flights would not have been closely recorded, especially the significant ones including trans-sonic and supersonic ops. Despite the best design and air-tunnel work, you'd expect that things would go wrong and you really want to learn as much as possible from any incidents/events.
Unfortunately, all this happened well before the internet age, and so records and images are not so easily found :(
I wasn't on the engineering team ;) but apparently they planned 4000 hours of test flights. https://web.archive.org/web/20150316210132/http://aviationwe...
It's almost inconceivable that the test flights would not have been closely recorded, especially the significant ones including trans-sonic and supersonic ops. Despite the best design and air-tunnel work, you'd expect that things would go wrong and you really want to learn as much as possible from any incidents/events.
Unfortunately, all this happened well before the internet age, and so records and images are not so easily found :(