According to ZTL, ZSL London has the following species found in only 0-5 ZTL collections:
Acigöl killifish, Develi killifish, Chapultepec splitfin, Balsas splitfin, polka-dot splitfin, eyespot rasbora, blackstripe rasbora (only ZTL collection), blue-striped angelfish, midnight angelfish, barred angelfish, Bleher's rainbowfish, grime rainbowfish (only ZTL collection), western rainbowfish, Charco Palma pupfish, Potosi pupfish, La Palma pupfish, checkered pupfish, Mezquital pupfish (only ZTL collection), boxer pupfish, discus tetra, flag tetra (only ZTL collection), flagtail butterfly tetra, stoplight tetra, garnet tetra (only ZTL collection), gold tetra, threespot leporinus, Y-bar leporinus, bannertail catfish, golden saw-finned goodeid, rainbow goodeid, darter characin, yellow-coloured dwarf flagcichlid, striped dottyback, blue-striped dottyback, staghorn damselfish (only ZTL collection), redface mac cichlid, Kendall's Tanganyika cichlid (only ZTL collection), striped Barombi Mbo cichlid, Tanganyikan cichlid (only ZTL collection), Hawaiian squirrelfish, greyface moray, mangrove rivulus, Monterey platyfish, rhino catfish, ornate pim catfish, flagtail catfish, orangespot metynnis, silver mylossoma, Arabian toothcarp, Roberta's toothcarp, pink corydoras, marked corydoras, red-tailed brycon (only ZTL collection), unicolor coralgoby (only ZTL collection), smudge spot cory, black acara, Tambraparmi barb, blackbelly tilapia
Congo caecilian (only ZTL collection), two-lined caecilian (only ZTL collection), Sardinian brook salamander, common midwife toad, Iberian midwife toad (only ZTL collection), Lake Oku clawed frog (only ZTL collection), Fea's treefrog
Big-headed turtle, tree runner, Fiji short-crested iguana, Round Island skink, Amazon Basin emerald tree boa, Ethiopian mountain adder, Central American bushmaster
Black-necked weaver, splendid sunbird, scarlet-chested sunbird (only ZTL collection)
Southern tamandua (only ZTL collection), aye-aye, potto, Moholi bushbaby, Australian water rat
Quite a few of those fish are common in the private trade, I always have found it interesting how so many fish species are considered rare in zoos but are abundant in aquarium stores...