Can anyone explain to me how filling the void in the fipple, say with Blu-Tack (UK Brand) or wax, can improve a tin whistle's sound or playability? Thanks.

I wouldn't, it's not worth it

I wouldn't, it's not worth it when a better whistle is still very cheap. The thinking is that the tone is coloured by the weight of the fipple, so increasing the weight darkens the tone by making it less resilient. But when you're playing a melody instrument, why would you want to do that? The original idea comes from noticing what happens when you clean an instrument you've played for a very long time, and where there's been a general accretion of crud on the tongue. What is intangible there is how much the instrument's already settled into the player's style.

Filling that void is supposed

Filling that void is supposed to cure a "mini vortex" that is formed as air rushes past. Filling the void is to clean up the sound. I know this much, it's very difficult to get the filler way up in there properly.
Good luck!
Rich

Rich