I don't see anything wrong with a PPL flying an ex mil fast jet on a permit. It depends what type of flying of course.
There is too much to go wrong. The pilot of the crash reported below did not have the skill for what he was attempting. You can argue that the Shoreham pilot had the skill, but still - it went wrong.
https://www.gov.uk/aaib-reports/aaib...-t-mk-1-g-timm