Unfortunately having spent the last 10 years in the acquisition stream covering T&E, research and finally the last considerable stint at DE&S, getting 'fit for purpose' is incredibly difficult because no-one is willing to underwrite (or even define) capability level KURs.
The MOD is very much risk averse when it comes to commercial decisions, and as a consequence would rather roll over than play hard ball with industry. MOD tries to play fair with EU procurement regulations and then wonder why we get shafted as everything takes so long to define, compete, procure and deliver.
An analogy which has many real-life examples: imagine you are a company that wants to procure a new engine management system for your fleet of BMWs, you would think that you could just go to BMW and buy new ones. In the MOD, a case has to be written to say why you can't go Ford, Audi and Bentley to get a new engine management system for your BMW. You have to do this in formal commercial terms through advertising in the journal, pre-qualification questionnaire and maybe a full bid (because only at the bid stage is money allowed to be discussed). That takes time, money and people to do that work. Or it takes someone with big (political) balls to say "No, we are going this way" and take the informed and accepted risk.
Now in the case of an overseas capability, how do you get an overseas (or even UK contractor) to sign up to any performance of mission critical capabilities which are UK-specific such as DAS, comms or weapons. Furthermore, how do we programme, test and accept such capabilities if they are US systems where we are not allowed access to the guts of the system or the T&E data?
Whilst I accept that an off-the-shelf cost for Block III might be cheaper, TLCs need to be considered when looking at the niche capabilities we have rather than the helicopter itself. Or, we just accept that the US T&E system works perfectly fine and they get 'fit for purpose' from their platforms when delivered to the front-line...
No easy answer, but blindly trusting the US to deliver to spec (when they have defined the spec which will not be the same as ours) is potentially foolhardy.