The disk space was actually more restricted than 100K.
The original disks (if you bought one) was marketed as a "Dual 40/80 track disk" that would work on a single stepped 40 track drive, and both single and double stepped 80 track drives.
IIRC, it did this by fiddling with the number of tracks that were recorded, and having 6 of the 80 tracks (or three of the 40 tracks) near the beginning of the disk that were non-standard formatted, which allowed it to work out what the type of disk drive was for. I think that the track number was actually written into the sectors, allowing the program to seek to a track, and read the track number, which then allowed the program to work out what type of drive it was.
This meant that the program actually fitted into 37 of the 40 tracks.
As a side effect (or maybe it was the original intent), this made normal disk copiers fail to copy the disk.
Again, if my memory serves me, these 6 tracks were formatted with 128 byte sectors rather than the normal 256 byte sectors.
I remember that Acorn User published an advanced track-by-track disk copier that directly accessed the 8271 controller registers to implement a 'universal disk copier', although it assumed that the disk format was uniform across the whole disk (it checked the sector size and number of sectors on track zero, one and two, and then applied the format to the entire disk).
I took the basic 8271 programming procedure from that article, and used it to analyze an original Elite disk, and then wrote my own track-by-track copier that read the format and data of each track, formatted the same track on the new disk with exactly the same sector sizes, and then wrote the data. It was horrendously slow, because it only worked a few tracks at a time because of the limited memory in the system, but it created 100% playable Elite copies (and I believe that it would have coped with any valid or even some invalid disks, as long as they kept within the capabilities of the 8271).
The 8271 was an obsolete chip before Acorn chose to use it in the BBC micro. and it had severe limitations. The WD 1770/2/3 controllers were significantly more capable, but weren't compatible. They did appear in later BBC Micros (I think in the Electron and either the B+ or the B+128, and all later systems). Some vendors who provided 1770 controllers for the original BBCs also built a 8271 emulation into their adapter and DFS that allowed programs that directly accessed the 8271 (like the original Elite disks) to work, but not all did that, meaning that old versions of Elite would not work on BBC micros with some vendors 1770 disk controllers. I don't believe that the Acorn 1770 DFS did, making it a poorer (even if it was an original Acorn design) version than some of the third party ones.
Of course, later versions of the Elite disk would work with WD1770 or Intel 8271 controllers, and I think that Acornsoft actually offered a disk-swap for people who upgraded their machines.