If London Zoo had a limitless budget, then I would use those two acres either to build an Arctic-themed complex with underwater viewing, cold-climate fish and multiple species of pinnipeds (somewhat like Hagenbeck’s Eismeer) or a world-class, spacious, densely vegetate tropical house (akin to Bush and Masoala).
On a more realistic budget, I would opt for something more simple - an African Savannah-type exhibit. Two acres isn’t enough to create anything world-beating, but it would offer the giraffes, zebras and ostriches a substantially better home than what is currently offered on the Cotton Terraces, while also allowing for the addition of some interesting antelope (wildebeest would be wonderful) and a rhinoceros, a much-needed crowd-pleaser. The onshow stables could, in Colchester or Magdeburg style, feature some additional displays of their own. This would be an excellent chance to bring back Aardvarks, add more aquatic displays (it seems as though a single devoted aquarium is not on the cards, but a selection of smaller ones throughout the zoo, such as the one that already exists in Tiny Giants, would be a good way to bring back fish in large numbers), and bring in some entirely new additions - hyrax and weavers, for example, are both species that I would love to see at London, and would fit in well in such an exhibit.
I certainly hope London can negotiate an expansion into the surrounding, currently unused areas, of Regent’s Park. Perhaps if it is more accessible, expand across the Outer Circle into Primrose Hill, although that would require another series of tunnels or bridges. Either one is unlikely, but you never truly know.