I can't relly see the point in spending £4 million or so on a tiger enclosure, when tigers are already over-represented in zoos and have been saved from extinction. ZSL could always send Whipsnade's Amur tigers to another zoo and move the Sumatran tigers into the enclosure made vacant. There is scope to keep pinnipeds at London and there are several species of small cats and other carnivores that are more in need of conservation than the ubiquitous tiger.
I dunno when you have been reading lately ... but no tiger subspecies has been saved from extinction to date. Despite decades of good conservation effort most ssp. are still way on the brink. So, to not devote space to tigers ,,, which incidentally people come flock to see ... you creating your own bubble.