First of all, the pictures of the British soldiers were taken early in the war. Even though it was officially adopted on paper in 1939, the No.4 rifle didn't really start to replace the SMLE until roughly mid-way through the war; quantity production only started in mid-1941. Given the quantities required, it would not be possible to switch over to the new rifle overnight. Different theatres also had different priorities for issue of new equipment.
Second, while the magazine cutoffs and volley sights were superfluous parts that were omitted from production, they didn't really detract from the utility of the rifle, either. There was simply no good reason for the old version already in the system not to continue to be used as-is alongside the ones without the extra parts.
Third, the P14 had a good reputation for accuracy and its shortcomings relative to Lee Enfield rifles were not as important when used in the sniper role. It thus made perfectly good sense to use these already-produced rifles in the role, for which they were best suited.
Just curious, what do you see as the shortcomings of the Pattern 14? It was supposed to make the SMLE obsolete, until WWI got in the way. They did carry over a very similar sight arrangement onto the No.4, the only major advantage I can see is the extra five rounds in the magazine.