Well if we assume that Indo-Europeans had Alpine-like skulls, it would explain the widespread presence of the Alpine phenotype. I think that Celts were very Alpine, and I think this explains why both Southern Germans and French are very Alpine in appearance.
I would argue that many British who are descendants of Celts tend to have roundish faces. Even the 19th century stereotype of round-faced Welsh and Irish seems to suggest this. Perhaps, only Brits who are descendants of the Normans, Ango-Saxons, Danish vikings, etc. are the long-faced ones. Most of the British upper classes are long-faced and it is usually them that we see in the media, so I think that we are under an illusion about what the average Brit looks like. Most British folks that I have personally met have had fairly round faces. I also heard from one of my old college professors (who was Welsh) that the Welsh (and Englishmen who live around the area of Wales) tend to have significantly rounder faces than most Englishmen.