At last someone who talks sense.
I have worked on most of the UK's major airports over the last 10 years both on the ground and as pilot.
As you quite rightly say it is a nightmare when moving between different airports where they will not accept other Airport Authorities passes or qualifications.
I have lost count of the number of airside driving courses I have done all showing the same video and holding the same exam and the number of pass applications I have filled in asking the same questions.
And why is this you may ask? Money of course. Each airport wants their cut and screws the operators for every last penny they can get. So what if you have done the same course 5 times before at 5 different airports we didn't get our £70 or £100 worth is the usual answer to this question.
So yes it is about time there was at least a National ID pass which allows access to different issued by an authority who has the time and energy to deal with this rather than an Airport Authority who is just desperate to get a few more pounds in the bank.