I think this is the Royal Academy of Sciences way to admit that Physics as a research subject has ground to a halt. String theory suffocated theoretical high energy physics for nearly half a century with nothing to show for it, and a lot of other areas of fundamental physics are kind of done.
My sense is that we might have reached the limits of what we can do in high-energy or fundamental physics without accessing energy levels or other extreme states that we currently can't access as they are beyond our capacity to generate. From what I've read (not a professional physicist) string theory is not testable unless we can either examine a black hole or create particle accelerators the size of the Moon's orbi…
Although rare, there are cosmic rays that do span very high energies. You can access these from, for example, atmospheric showers.