No point in splitting up the herd, it is a veterinary issue at best. Besides, if you wish to contain the virus you much first isolate its source. Transfer or transport out would be the worst possible option or way forward.
What Whipsnade's curatorial and vet management need to determine is the source of the infection, which individuals in the herd have been exposed to EEHV, which ones are carriers and which ones are non-carriers and determine which individual may have been the potential source of the EEHV virus (not that this in itself would have any repercussions on this particular individual) infection.
Efforts need to be intensified to find an effective treatment for EEHV in Elephas maximus, both adults and infants (the latter category being the more vulnerable). If I am correct, several herds in continental Europe do have surviving calves and adults from previous EEHV infection. We need to take the research from there.