As I said here...
One male cleared the high fence with ease also- just decided to jump out one day. They were extremely nervous. The idea was to mix them with Giraffe etc but they never (afaik) left their hardstanding as it would have been impossible to get them in at night etc. I wish Whipsnade good luck with them but fear they may have the same problems if they try to keep them in anything but one single paddock where they aren't moved around. Even with minimum disturbance they can be difficult.
I don't think that, in general, the antelope at Whipsnade are forced inside each night. Those that are nervous are provided with shelter that they have the choice to use. This is definitely true of some species there.
Plankendael has a decent herd of impala that are mixed with giraffe. And these are brought in every night.
One of the first impala I ever saw was in a pretty small enclosure in the old Poznan Zoo in the city centre. Considering the confines of its enclosure, it did not show any nervousness at the proximity of visitors.
The Whipsnade animals seem to have come from Emmen. A batchelor group of five yearlings. The history of these can be traced back to original imports by Dvuur Kralove in the 1970s. A batchelor herd = an experimental group. If things go tits-up then no valuable breeding females will have been sacrificed. If things go well, then presumably there is scope for females in the future.
It ought to be easier to provide quiet space for Impala at Whipsnade than Marwell. It might help, too, if they were kept with their dens offshow. The latter applies to Blesbok too in my experience - although generally speaking, alcelaphine antelope seem very easily spooked by the close proximity of humans. Durrell talked in "Beasts in My Belfry" as to just how frighteningly nervous the White-tailed and Brindled Gnu kept at Whipsnade were in 1945.
Although relative flightiness is species-specific, the background of particular herds and individuals also has a major influence. As an example, Nile lechwe from Rome Zoo (where they are kept on a small hardstanding with very close public viewing) would behave totally differently to the same species originating from, for instance, Whipsnade (where they are basically feral and are never exposed the pressure of close proximity to people).
My own experiences with blesbok are entirely different. They are comparatively steady animals. They are not good jumpers either. They are better at ducking beneath or ploughing through obstacles rather than jumping over them. I am not sure how other hartebeest-type antelope compare.