Look at http://theokapi.org/welcome.aspx. It keeps records about all okapi in captivity (also in Epulu station in Congo). So you can find there information about all okapi in UK, their parents and grandparents, also their offspring (http://theokapi.org/Studbook/Who.aspx).
Unfortunately this site stop being updated too, around 2014 I think. A great pity as it was an excellent resource