There are a good few mint in box with all the little bits and bobs that came with the Centenary: stickers, sling, badges, certificates. They hold the premium well; £1k isn't robbing anyone though £850 is the norm.
Far fewer .177's were made, about 1/4.
Used or slightly used and without all the bits are few and far between. £500 is a fair price.

At the end of the day they were but a blinged up Airsporter S, and don't shoot any better. These point and shoot really nicely with open sight. The BSA branded Weaver?? scope doesn't add much to what can be acheived, they are not a HW77.
I think they are one of the prettiest air rifles, and BSA's, done. I think the Centenary handles better than the standard Airsporters, they give a real pleasurable shooting experience. Anyhow, I was sad to sell mine. Anyone buying one should be happy for what it is, a fine rifle.

Good luck with the sale. If you are not keeping it make someone else happy.