I am uncertain about the UK, but anecdotal reports of the Canadian
C1 rifles in the late 70's and early 80's was that they were worn out, and had reached their service life and in need of outright replacement. I am sure that BAR can likely elaborate if that assessment was valid or not as the C1 was the rifle of his service period.
If however the rifles were indeed in bad shape in Canadian service imagine that a similar thing might be reasonable for the UK rifles.
Our rifles were replaced in the early 80's with the C7 series, the modified M16rifle, I can imagine that another decade of service on a potentially tired rifle might be a long stretch to ask of it.
My contribution is merely discussion points, I am sure we will see a proper assessment from Peter, and BAR for the Canadian angle.