From memory, I don't think that full yellow ends were ever mandated for safety reasons and the traditional yellow warning panel carried when the then-new diesel locomotives were introduced would have sufficed. I have red that the full yellow ends were introduced as part of the 1960s corporate identity scheme. The very first draft of the scheme envisaged a wider, larger yellow warning panel than had hitherto been the norm, with the depth of the yellow area wrapping around the side but using the new bright BR Red, onto which the new BR double-arrow logo would be positioned. The only item ever painted this way as far as I am aware was the Brush Type 4 (Class47) painted and delivered this way for use on the new XP64 train which showcased the new corporate identity.
Before implementation of the full new branding, various 'tweaks' were made, including the decision to paint the entire ends of all locomotives and multiple units in yellow. This was partly to improve safety, but also to make the front ends of all BR trains as similar as possible as a part of the overall brand. This was maintained for many, many years until sectorisation gradually saw this mandate being relaxed.
I personally think the new stock without it look very contemporary, but how the fronts of trains will appear if GBR does eventually adopt a unifying brand will be interesting to see.