My guess is that the space gray Sport is one of the MORE popular models. It apparently is one of the more popular MacBook models, for instance (though the gold is the most popular). I'm sure you'll get it before May 8. If May 8 comes and goes then you have a legitimate gripe. Until I got the Preparing for Shipment update, I was fully prepared to wait until May 8 for mine.
Let's think about this further. What if there were an unexpected production issue that sent the space gray defect rates soaring? Do you stop production entirely until you sort it out, or do you crank out more of the other watches so that you catch up with your overall backlog?
Again, then why are they shipping some out in the UK? Are they just hoping people in the UK are more tolerant to defects? I doubt it.
Also your logic is flawed in the fact that if the model was more popular, then they would produce more, obviously, not less. Not even one has charged in the USA, so if they were making more, I doubt not even one would go out. I think the 38mm was actually the LEAST popular to be honest, so they knew it wouldn't matter if they screwed over just a few people.