There is a small animal collection nr Retford, N.Notts - Wetlands wildlife park - that has at least a couple of Yak.
Whipsnade did breed Musk Oxen. A pre war plan of the park shows yaks in Dagnall Paddock, the present site of the Cheetah enclosure. Whipsnade My Africa by Lucy Pendar mentions an unusual export of yak in 1989. During the USSR's invasion of Afghanistan in 1980 the Kirghiz people, (of Turkish-Mongolian origin) fled to Pakistan. The Turkish government consider anyone who speaks Turkish to belong to their country and organised a relief programme. The Kirghizis were transported to an area near Mount Ararat and allowed to settle there. When asked what it was that they missed most of all from their previous existance they replied yaks. ZSL were contacted as they had the best breeding record with yaks in the world and 1.2 were donated to the Kirghiz people. The male was deemed not healthy enough to be exported but the two cows were sent abroad.
That's interesting. I have no recollection of them in that paddock during the 1950/60's but maybe I never noticed them. I can remember when it contained a large herd of Red Deer though. Presumably the current Yaks are from the same continuous breeding. I presume Whipsnade's have always been pure black, though you can get other colours too- piebald and a 'brown roan' colouration too- I imagine black is a dominant colour though.
I've only ever seen black yaks. Dragon(ele)nerd. Musk Oxen are not very common in European zoos, I don't know about yak, I assume all the captive ones are domesticated anyway.
Antwerp Zoo also has Yaks. But I don't have recent stats about them. They're getting a new space this year (almost finished).