Looks like they are just buying enough for the combat arms people.
This is how the MOD thinks. Yes, SA80 is not the best, but it is useable. They need 50,000 rifles for the combat arms soon and it is difficult justify to have two different rifles in the system to the government, and they are running out of time. So they are wiling to basically buy 50,000 of the new "same thing". It is probably more "bang for the bucks" from the user perspective to get the newest but the 'corporate' view is they don't think it is worth the other money, the fight and other intangible/tangible problems, and there is not enough time
If the combat arms want their own more advanced weapons, is it doable? Hell yeh, the special forces and royal marines have their own Diemacos. Lots of countries somehow manage this kind of things. Upgrading and rebuilding the same thing is easier to justify by the MOD to the government ( look at the fiasco of the committee holding up the USMC's 20-% budget for the HK M27). When you propose a brand new solution, the justification is getting way more scrutiny and this means it is not getting done onetime . Advocating for new "dual" small arms system within the MOD doesn't seem like an essential career move either with high risk of failure, and there must be enough internal advocates for it. Either they were never there or got rolled over internally early on.
This really means the UK armed force will be using SA80 til 2030. Whatever the US wants to do with 6mm in the next 10 years won't begin to filter down to the rest of NATO until the 2030, probably not to France and Germany until 2040. We will be all retiring by that point.