There are a lot of bad whistles made. One common problem is to use a tube that is too large in diameter. This makes the Whistle sound stronger in the low register but puts it painfully flat in the second octave. Another common problem (for which there is no excuse) is to make the holes the wrong size and in the wrong place. Particularly common is for the low D note to be grossly sharp.
Generation D Whistles are a good compromise for price and quality but in my experience intonation in the top octave is improved if you fill in the little space underneath the windway with blue-tac or something similar.
Remember if you have a good Generation top you can make your own body from readily available (in model shops) brass tubing. It's not rocket science and you'll learn a lot about intonation on the way

All the best PP.