Let's suppose that post WWI Britain worked with France to develop a new common rifle round resulting in the OTL 7.5mm round. Britain then needs new weapons with which to shoot it. It can and probably will adapt existing weapons to begin with, but that would only be a stopgap. I'd expect the Bren to still emerge as Britain's new LMG but a medium machine gun and rifle will also be needed, what are the best options?