Please dont sniff at Wildlife Parks as you can watch deer for example at a close range and pick up so much information about their way of life etc. that would take years of study in the wild.
The fact that at certain times of the year the stags/bucks split into sexed herds and what seasons they get back together, how mums protect their young, and how to tell apart the different speicies, what colour they are in any particular season. How the deer of different age groups behave differently within the herd, when the velvet comes off, and the alarm bark from a fallow doe is well worth hearing on its own, and I havnt even mentioned the rut yet.
I think there is as much to be learnt at a wildlife Park in Britain as there is being dumped out of a land rover to within 10' of a sleeping rhino then driven to within 12' of a zebra in the dusty Massai Mara. You certainly wouldnt need a long lense.
But thats just my opinion
Regards
Rob