Seat upholstering...help! :)

They almost look like rivets?

If they are, small drill in the center hole will get them out.

Brian is right sure looks like a rivet to me. 

Yup.  Use a suitable drill to grind out the center of the rivet and the collar will pop off so you can then remove what's left of the rivet, and the carpeting may then release, but I doubt it.  The carpeting may be glued to the seat back with some sort of spray-on adhesive, like 3M Super 90 or something.  If it is, just gently keep pulling on it and it should release.  A gentle heat gun or hair dryer on high might help, too.

John - Looks like UK replicas are a lot different US versions.  On CMCs the carpet is just glued to back of fiberglass seat shell - it isn't even bound or does it have and edging.  Yours looks like carpet has edging and is glued to some thin backing then riveted to the fiberglass shell.  You might be better off with what you have as the carpet could be reused.  On CMC the carpet is probably sacrificial once glued with contact cement.  I'd start with 1/* drill in center and then go larger - careful not to catch carpet in the drill bit.  

I like the replacement 2 tone seat covers - are they custom?

John - Looks like UK replicas are a lot different US versions.  On CMCs the carpet is just glued to back of fiberglass seat shell - it isn't even bound or does it have and edging.  Yours looks like carpet has edging and is glued to some thin backing then riveted to the fiberglass shell.  You might be better off with what you have as the carpet could be reused.  On CMC the carpet is probably sacrificial once glued with contact cement.  I'd start with 1/* drill in center and then go larger - careful not to catch carpet in the drill bit.  

I like the replacement 2 tone seat covers - are they new and custom? Or is that how baggy the old ones got?