I'd suggest doing some searching through this forum, and looking at the PFA's own site which although difficult to navigate does have an explanation of the differences.
Jets, Hurricanes, Spitfires and the like are outside the limits set for a PFA permit - they fly under CAA permits and incur all of those costs.
With a permit you can fly in CAS, you can also visit almost all European countries without any issue (although some do require prior permission - again see the PFA site).
As to instrument flying - what a joke, as approximately less than 0.5% of PPLs actually have an IR then it is pretty unlikely this would ever be an issue. Cruising at FL120 - there are a handful of aircraft with oxygen systems and the performance on the UK register. If you want to do that then that is serious flying and pretty much academic if you need to know the difference between C of A and permits!
Spend some time on the PFA site and then ask more focused queries.