If the 'mouthpiece' is removeable (as with the 'Generation' whistles), you can sometimes trim a couple of millimetres off the tube length, thus allowing a little more leeway in tuning, but it's drastic and likely to be of little use.

I, too, have an Eb Generation whistle, and a low Bb - which will, as suggested, play in Eb - just as you can play in G on a D whistle.

I have (I think) a full set of Generation whistles, which start at low Bb, C, D, Eb, F and G - and includes a three-hole Tabor Pipe in D which looks like an ordinary D, except for the absence of some holes...!

My tunable low G tabor pipe is something of a rarity, as Generation stopped making it some years ago (mine is 35 years old) - and it doesn't have a plastic mouthpiece. It's entirely brass.

Many keys are accessible from other whistles by learning 'cross-fingerings', but you have to pay more attention to the blowing pressure to stay in tune, which makes it hard work...!