Beyond that range heavier weapons and supporting arms come into play and so the .270 was seen as the ideal and also you needed 'less gun' to fire it and it was easier to fire it on fully automatic
Not quite actually. The Ideal Calibre Panel initially suggested that the ideal calibre for the British army was between .250 and .270.
However, the two calibres presented to the General Staff were the .270 and .280. Both were actually a bit too big as far as the ICP’s recommendations go, but the Staff had already been debating adopting 30.06 due to budgetary constraints and the fact that British industry couldn’t, at that point, completely replace the weapons required to equip the British army. It was only lobbying from those involved in the program and the news that the US was reconsidering their main round that caused the Staff to back down and let the ICP continue. So the round presented needed to have a chance, at least, of also fulfilling American requirements. So both the .270 and the .280 were compromised by this requirement from the start.
In the (probably unlikely at this point) event that Britain feels comfortable in its ability to equip its forces, I think it would be more interesting if they developed this earlier:
6.5mm. Developed in the 1960’s by the same people who made the .280 and .270 before the Troubles diverted attention to much smaller ammo with less chance for collateral damage. It was initially tested using a .280 cartridge ( and is thus generally shown as 6.5x43). However, though I can’t find the post now, I understand from Tony Williams that the intention was to develop a slightly longer (50 mm?) slimmer case for the actual round.
It should be noted that this was not the first time that Britain showed interest in the .256 calibre. Even before the often cited use of Arisaka rifles in WW1 the requirement in 1908 that eventually produced the .276 was initially intended to produce a round in .256.
EDIT: The round developed in the 60’s was 6.
25mm not 6.5.