It looks like a platter from a DataProducts drive, as was fitted to Atlas at Manchester University in 1965, around the same time as the KDF9. The capacities match. If it is, it will be 31" diameter.
It looks like there's a bit of scoring about a third of the way out from the hub, but the data might still be readable with the right equipment. Modern heads are intended to float a LOT closer to the surface than in those days, but are much more sensitive. It might be possible to float one clear of the surface and detect the patterns.
I don't go back quite that far; I started work a decade later on an ICL 1901T machine with 3 x 60 MB drives. The platters on those were about 18" diameter.
In 1984 I set up my first file server using Microsoft software. It had a 5 MB drive!
There's an interesting aside about computer jargon in France. In their attempt to halt the spread of "franglais", new terms for all things computerish were invented, such as "gestion d'ecran" for "video driver". However, sizes of screens and disks were still measured in inches, because there is a french word "pouce" for an inch. So there was the "disque souple de trois pouces et demi" for a 3.5" floppy disk. They missed a trick, though, because 3.5" is the rough imperial equivalent of EXACTLY 9 cm.