Flying training attracts big discounts which almost certainly applies to your experience at Benson.
Regarding Yeovilton Air Day - the display organisers are contracted in although there are some uniformed staff on the team. The landing fee fiasco you encounter is driven by the MoD not Yeovilton or the display organisation. A waiver can be given by the MoD if the Stn Cdr asks for it. This is useful if your aircraft is worthy of static display.
There have been various incarnations of fees at displays in recent years the worst example being that the 'RN' read as MoD, charge landing fres to the Vulcan at an horrendous rate to display at their own event. Multiplied as Vulcan operators wished to operate locally to Swansea and return for another set of landing fees.
The RAF took a pragmatic approach to this at Cosford that concluded with all air show associated aircraft enjoying an MoD waiver; but it took staffwork, and the organisers at Yeovilton are civilians.
As for benefits to veterans. As has been said, locally employed Servcemen get at best discounted tickets for family and friends. It is not an 'Open Day', it is a busines.
The Army & Navy get 50% discount on RAF assets for displays. The RAF do not! Go fiqure that one!
FWIW I would have put you in the static park, no fees nor costs, given you some accomodation, and a gesture towards fuel costs ie 50ltrs. But I do not work for Yeovilton ;-)