Mechta
It might be the case that they had been considered uneconomic to repair but the MoD are usually inclined to pay top dollar for defence related stuff so I suspect that is not the case, add this to the general lack of composite knowlage in the business and you see a picture of aircraft getting written off without good reason, you do have to remember that in 1990 composite technology was not understood outside a few glider repair shops.
The glider business is accustomed to glueing aircraft back together but powered aviation is by and large stuck in the metal frame of mind, off the top of my head I can think of about six powered aircraft written off by insurers that have been repaired and shown a tidy Proffit in doing so.
I will second your statement about composite skills being in short supply, one of the companies trying to get the lions share of the glider recovery project is offering wages £10K+ below the current market and to my mind are demonstrating they don't understand the cost of the skills needed to carry out this work, it follows that if they get the contract this very sorry farce will continue due to lack of skilled manpower.