The TG21 is probably quite sensible for annual camp and, I suppose, the same reasons could be quoted for any overnight stay. For a trip to the RAF Museum it is a ridiculous hoop to have to jump through every time.
We actually get cadets to put a NoK contact number on the electronic sign-up sheet for activities which can either be printed and handed to the staff member, or accessed by staff members from their smart phone. I can't really see the need for the GP's address on a day trip (unless you have 6 weeks spare to make an appointment) and surely cadets are capable of letting us know their own dietary and religious requirements (which, interestingly, before my redraft of the form some years ago, were listed under medical requirements!). They usually bring a packed lunch anyway.
The annual updating of 3822A would put a paperwork burden on an already stretched squadron admin system, unless it was merely a matter of sending the old data to the parents and asking them to amend if there are any changes (we can then assume no changes if there is no reply).
From where I am standing, the whole system smacks of HQAC staff never having seen a cadet