A few points:
As Mach has said, the UK military has had quite a few incidents and accidents at displays and during display practises over the years so their system is not infallible. I believe that one weak link in military displaying is that often the supervision duties are allocated according to rank, and the supervisor may have no display experience whatsoever (that certainly was the case for my first military display season); at least all CAA approved DA Evaluators are experienced display pilots with relevant experience in category. And I believe that the fact that, except for teams such as RAFAT or flights such as BBMF and RNHF, UK military display pilots only have 1 or 2 seasons during which they display is a driver for some of the recency requirements. Many civilian display pilots have displayed every season for several decades and do not need as much practise or recency.
That they allowed someone to achieve a display qualification on a straight-wing basic trainer such as the JP as a means to display a Hunter is professional madness - yet this is what the CAA did, whilst billing people for the privilege.
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That is not quite true. An initial issue DA for the Hunter has always required an evaluation on the Hunter, and a DA would then only be issued for that type following a satisfactory demonstration of ability to an appropriately certified DA Evaluator in accordance with the requirements of CAP403. However, at the time of the Shoreham accident a pilot could renew his DA in any category/on any type for which he held a DA and that would renew the DA for all categories held. Therefore, a renewal on a Jet Provost would renew a DA for the Hunter if the pilot held a valid DA for both types. CAP403 has been amended since the Shoreham accident and the rules for DA renewals have changed.
Some of the pilots who display historic jet aircraft do also fly military registered fast jet aircraft as a 'day job' and, therefore, what may appear to be low overall fast jet recency may not be the case. However, I do accept that these are a minority of display pilots.