Sorry if this has already been discussed, I couldn’t find it. 
I am thinking about getting a new whistle soon, right now I have a Clarke original D. While browsing whistles I couldn’t help but notice the amount of tuneable whistles. How do tuneable whistles work? Are they in any way better than non-tuneable? Are all two piece whistles tuneable? 
I am not pulling your leg, just a totally confused amateur. 
You can tune a guitar, you can tuna fish, but how do you tune a whistle? 
If you’re going to be playing a lot with other people, a tunable whistle is pretty helpful. A tunable whistle basically has a slide that allows you to alter the length of the instrument by a few centimeters in either direction, thus sharpening or flattening the whistle. If you’re just playing with guitar/fiddle, they can tune to you. However, if you’re playing in sessions, it’s more respectful to adjust your own tuning rather than make others tune to you (especially if a relatively non-tunable instrument like an accordion is a little sharp). If you’re a solo player, then you don’t really need one.
Jerry Freeman’s whistles are moderately tunable, as are Susatos, Feadógs, some Dixons, etc. In the more expensive range pretty much everything is tunable. Burks probably have the most precise tuning slide, and Busmans stay in tune over a wider range of potential adjustments.
Does that answer your question?
Ah, yes.
A tune-able whistle is a little bit of a misnomer. But if you are going to play with other musicians it is advisable to have one. If you understand the physics of the whistle you probably have figured out that by merely changing the length of the tube you can adjust the bell note but since the overall scale is fixed by drilling the tone holes a length adjustment does not really re-tune the scale. The intervals between notes remain fixed. But…and this is why it is important…it does let you adjust your pitch so that you are in the same neighborhood as other, perhaps non-tune-able, instruments. So in practice what you would do is before playing everyone tunes to say the A note (or maybe G). So you know you are at pitch in the middle of the instruments range and you have to blow the rest into tune as you play. It actually is useful and works well in practice.
In theory all two piece whistles should be tune-able. But there are some interesting issues that arise from where the separation between the two sections takes place and what mechanism is used to make the joint. If opening the joint results in an expansion of the bore of the whistle it can cause significant tuning problems depending on where that expansion takes place. Without going into particulars lets just say that a good tuning joint should not result in a significant expansion of the bore. So if you make a PVC whistle with a thick wall tubing and just cut the tube into two parts and connect them with an outside collar like a pipe connector, you could get some whacky scale results if you happen to place the joint in a less than optimal spot.
Hope that helps and does not confuse much.
Feadoggie
Thank you! That explains a lot. 
And, FWIW… many of the cheaper whistles can be made semi-tunable by soaking the head in hot water for a minute or two and loosening the glue so the head can be removed or moved around. I’ve done it with a Walton and a pair of Gens (couldn’t get the head on the Sweetone to budge, though).
The Sweetone head is not glued, just tight because of the seam. Wear rubber gloves for grip, and twist it in the direction of the seam*. Should twist right off.