It is been 24 days since you could pre-order an iPhone 6s in the UK and 11 days since you could walk into a store and buy one.
It's interesting to note the current availability of specific model types. If you want a standard 6s, you can walk into any store and buy one without queuing, you can reserve one for any slot or order online for delivery within 48 hours.
The picture is very different for the larger 6s plus. These are completely out of stock in every flavour (even the normally unloved white 16 GB) with what looks like a 3-4 week back order.
Given the supply chain data that was available to Apple in the 12 months prior to this launch, they would have had a pretty good idea of what the size split was likely to be - based on sales of the two sizes of the standard 6 models.
Whilst it's possible that a specific component of the larger phone may be supply constrained outside of Apple's ideal plan, I doubt this, given the more stable run in to the 'S' cycle model.
The disparity that shows they have underestimated 6s plus sales and over estimated demand of the standard 6s leads me to conclude that the speed of switching to the biggest phone is accelerating faster than Apple (and most tech commentators) expected.
The 'largest phone on the block' arms race appears to be far from over.