We asked a man at a poultry show who was at the D'Uccle Club table. He said that "here" (meaning USA I assume), we say Dee-uh-kul. But the actual pronunciation should be Dee-oo-clay, since it is Belgian. I have heard it pronounced Dee-uh-kul...I have actually never heard anyone (around here) say it the other way. It appears many of you here DO say it properly!!!
Pronunciation will be dictated by whether or not there's an upward-slanting accent on the 'e' of "d'Uccle".
I did a search on www.google.fr to see how it's officially spelled. It appears to be "d'Uccle", without the accent. That would mean... if the Belgians aren't as dumb as the French joke... that the pronunciation is "dew-kluh".