Captain Laidler has addressed this issue most succinctly in prior posts. Due to the design and harmonics of the No.4 Mk.1 rifle chambered in British .303, the most accurate form of bedding was the one that was they had. Experimentation with other forms of bedding did not produce the same level of accuracy. I am sure that they would have cut back on material if they could (and there were experiments with lightened rifles). I do not know much about No.5 Mk.1s (heck, I don't know much about No.4s), but there is a reason why the rearsight was only marked up to 800 yards versus 1300 yards. Regarding the L42A1, this was chambered in 7.62x51 and had a heavier barrel, which changed the harmonics and allowed a shortened forend.