ASSAULT RIFLES AND THEIR AMMUNITION: say this
The EM-2 was a carefully-judged attempt to produce a weapon which could replace both the 9mm Sten SMG and the full-power .303 Lee Enfield rifle in one compact package. A GPMG based on the Bren mechanism but with belt feed, the TADEN, was also developed to use this round and replace both the Bren and (at least partly) the Vickers MMG.
The EM-2 + 7x43 combination appears to have achieved all that was asked of it, and in 1951 the cartridge was briefly adopted by the UK as the '7 mm Mk 1Z', at the same time as the EM-2 was adopted as the 'Rifle, No.9 Mk 1'. However, it faced insurmountable political obstacles. Previously, it had been submitted for comparative testing in the competition to select a new standard NATO rifle/MG cartridge. The 7x43 was regarded by the US Army's testers at Fort Benning as a better basis for development than the new US .30 cal round with which it was competing, and other NATO countries (Canada and Belgium, at least) were very interested in the concept. The British and Belgians made great efforts to meet the objections of the US Army, who thought it wasn't powerful enough, first by stepping up the loading to 2,700 joules, then by developing a longer cartridge (the 7x49 - which actually saw service with Venezuela in the FN FAL rifle). Despite this, the Americans insisted on NATO adopting a common round which had to be of .30 calibre and powerful enough to replace the .30-06 in MGs - which meant by definition that it could not be used in an assault rifle. A change of government resulted in the British giving way and cancelling the EM-2 and its cartridge in favour of the FN FAL in 7.62x51 NATO, which apart from being half an inch shorter than the .30-06 cartridge represented no progress whatsoever over this fifty-year old design.
The TADEN was a British experimental light machine gun firing the .280 in (7 mm) intermediate round. Alongside the bullpup EM-2 rifle design, it formed part of a proposal to reequip the British Army with new small arms which would use a round smaller than the .303 inch which was shown to be impractical for use in a modern assault rifle.
The Taden would replace the Bren gun as the light machine gun and the Vickers machine gun as the medium machine gun. The Taden used elements of the Bren. The EM-2 would replace the Lee-Enfield rifle and 9 mm submachine guns.
my opinion is that the UK and for that matter the Commonwelth would still be uesing bothe, and only now looking for a new type. it would be hard to say how the EM2 And taden would have changed things other than a slight change in infantry tactics.