The Buzz aircraft are operating under the 'Ukay' callsign until the end of March. As part of the sale of Buzz to Ryanair, KLM had to switch all of Buzz to a new company name (Buzzaway Ltd) and had to get a new AOC (or whatever the licence thing is called) for the new company. Therefore if Buzzaway Ltd are using 'Ukay'; KLM can't.