Of course WHL flight crew are involved in development
WHL use a joint approach with TPs and FTEs each having an input during the design and devlopment process, WHL use those inputs like any other company. the types of testing that useful feedback can be generated on are fairly self explanatory (handling, vibration, HMI etc)
Unlike some companies WHL have no real distinction between experimental/development/production test crews, all FTEs and virtually all pilots do all aspects of the test job, mainly based on experience, ie I believe the more experienced the individual the more likely to be involved with the experimental type stuff. on the other hand all get involved with production testing (which appears to be pretty run of the mill)
From memory the FTEs represent the core engineering view with the TPs putting in the handling and in-service view of the product, all the TPs are probably ex UK military services whilst most of the FTEs are not(it seems to work OK).
(it appears to be a British trait to remain quiet on a subject like this which is why you won't get many replies)
DM