The reason koalas dont do well in the UK is that they are extremely picky eaters. They will eat about 30 of the 650 eucalyptus varieties,which isn't highly nutritious and these varieties contain less toxins than the others. While the koala is adapted to deal with these toxins, if the leaves aren't fresh the toxins will build up in the leaves and make the koala sick (usually they just wont eat them though). Koalas do eat a small amount of other vegetation such as gum tree leaves as they also get their hydration (water) from leaves rather than drinking.
In 1989, two female koalas (Mije and Billi) arrived at London Zoo, from San Diego Zoo, and settled happily, the eucalyptus was flown in from Australia or sent up from Devon. After quarantine they were housed in the large but rather stark circular room in what used to be the old orang house, and here they would climb on a wooden climbing frame against the painted backdrop of a eucalyptus forest. I remember being at the zoo at this time and they weren't well advertised/promoted so I was the only one in the house, it was certainly like they were exhibited as a second thought. One of the koalas became ill in 1992 and died. The Zoo considered it unfair to keep the remaining koala on her own so she was sent away to join a group living in a zoo in Portugal. The original koalas of London, almost a century ago, were fed on eucalyptus cough pastilles and these were the first koalas to be exhibited in England for nearly 80 years.
P.S. Don't like Panda Bear OR Koala Bear - its just not necessary, is it because these are popular soft toys?