According to Zootierliste London has held the following species:-
spotted-necked
smooth-coated
Eurasian
Canadian
African clawless
and the current denizens, the "infamous" short clawed otters.
I was expecting more myself, maybe not giant but hairy-nosed, neotropical or Southern river otter at least.
Giant otters don't seem to have been kept much until quite recently so there's still time for ZSL to get in on the act...