"parts interchangeable, that is a critical aspect when you have to deal with logistics during war time."
Actually, it wasn't as critical an issue for the British as it's made out to be...
First, for the British, having enough rifles was a critical issue. They had lost nearly half a million rifles in France. Many of those were the older No. 1 Mk IIIs, which were not interchangeable with the No. 4 Mk I.
Given the choice between having rifles in hand, but which had some differences in parts, vs not having rifles in hand while waiting for the contractee (Savage Arms and Long Branch in Canada, primarily) to change their tooling to give 100% compatability, they wisely took the "we'll deal with slight incompatability issues" approach.
The fact that other Commonwealth troops had No. 1 Mk IIIs wasn't an issue, because they supplied their own spares.
Regarding sights on the 1903 vs M1 Garand...
US forces used relatively few 1903 style rifles during WW II (those with the original 1903 ladder/tangent sights). Most 1903s fielded in combat were the 1903A3 variants produced by Remington and Smith Corona, which had the considerably better receiver peep sight.
That peep sight compared very favorably with the sight on the M1 Garand regarding accuracy ability, durability, but it wasn't as easy to adjust quickly.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.