What you probably need is a decent selection process that finds individuals with the raw material for those skills, and a 9 month training pipeline that hones them.One has to hope that this debate has already taken place at Andover, although I doubt it has.
The task - FID, in US terms (Foreign Internal Defense) - is about raising, training, mentoring and accompanying foreign regular and irregular forces. The US see this as a core SF mission (for the 'Green Beret' Special Forces) and not, for, say, the Rangers (who are highly-trained airborne light infantry with some 'SFSG' tasking) or any of the other multiplicity of US Special Operations Forces. The final graduation exercise on the USSF Q course, the basic qualfiication course, is still built around a FID scenario.
The Brits have form for this, with the Jedburgh teams, Force 136 and, of course, the SAS in Italy and France after the end of the North African campaign, ending up on mutli-squadron deployments in the Vosges and Northern Italy for example, to support and cooperate with Maquis and Partisans. These operations were expensive, difficult and potentially very dangerous - the capability, once deployed on the ground, is brittle and when things go wrong, the go very badly wrong.
If the role is as described, those performing it will require, not just high-level instructional skills, but also the ability to present themseves to their students and local partners as exemplars, in terms of fitness, skills and courage. They'll also need a good dose of linguistic aptitude and the ability to sustain themselves in likely austere circumstances over long deployments. Strong medical skills also, one assumes, as well as field engineering, demolitions and the like.
All that suggests to me that those performing the role will need to be substantially more skilled and trained than might be the norm, even in a Specialised Infantry Battalion. They will need personal characteristics which, again, will not necessarily be held by the majority of soldiers. SOme form of selection against characteristics and aptitudes is inevitable, if this isn't to be another Potemkin village of a typical British Army bugger's muddle.
Whether selection needs to be UKSF or Pathfinder-style is beyond me to assess. Other, better-qualified types will have a view. I just hope it's been solicited and this isn't being run by a jolly good sort cavalryman SO1 and grumpy Parachute Regiment OF-5 somewhere who got straight on the phone to Hereford when they were tasked.
What you could then do is fold into (pre-existing) US confirmatory exercises held by US SF to check you meet the standard.
Or you could select by making people run 6 min/miles and do the BPC.
Up to the internet I suppose.