I think you'd have a problem with the clubs. The paid instructors get paid little enough already and they'd be up in arms if they saw others doing "their" jobs for free. I suspect you'd also have a problem if the PPL/CRI was doing it for free and the club was charging for it (which they would be entitled to do as they would be providing the facilities behind it). There can't be a huge demand either for the services of CRI's. If you have a club with 500 members and they each need a revalidation flight every 2 years that's 5 flights a week to be shared out betwen the FI's and the CRI's.
Details of the PFA Coaching scheme are
here. I believe the take-up has been disappointing.
The cost seems to be around 30 pounds per flying hour for the coach, quite how the legalities over payment are met I don't know.
You can also do the revalidation flight with a PFA Coach, details
here.
Interesting that they say the coaches and assitant couaches can sign the C of E, as my understanding was that it had to be signed by an Examiner.
Mike