OK we need to be careful here in distinguishing the use of the histogram on a DSLR and as part of levels in Photoshop.
On the DSLR, it simply provides information on where the pixels are in the current exposure - you use that information to adjust the exposure (eg. adjust the peak from left to right, avoid clipping highlights etc.)
(or similar) the histogram is associated with an editing function (levels) - which allows you to redistribute the pixels. In the Britannia example, what has been done is to alter the contrast
not the exposure - by bringing the sliders towards the center, you are resetting the black and white points and boosting contrast by compressing the tonal range. If you were to save the Britannia shot, then re-open and check levels, you will see that the histogram has now spread to fill the graph.
Of course this can be the correct thing to do, depending on the image, but do notice that in the original Britannia shot, there ARE pixels at the extreme right and left of the graph, just not very many - by bringing the sliders in, subtle tones in the white and black end of the scale have been set to pure black and pure white. While clearly the midtones have benefitted from the adjustment, if you look at say, the undercarriage, subtle detail has been lost.
The same thing has of course happend to the highlights, however in the case of the highlights, it is the landing light which has contributed to the pixel count, and arguably, this can be "clipped" with no loss of information.
I think I would have adjusted this pic with a similar use of the highlight slider, a less agressive use of the shadow slider, and given the image a little more life with a rightward adjustment of the midtone slider.
BUT this just goes to show there is not a "correct" setting, but rather its a matter of interpreting what is in the image and how that is represented on the histogram.
It is important to remember that the histogram is not in itself an editing tool, it is simply a report of what is going on.
Colin K. Work, Pixstel