The problem is that with a 100mm joist height you aren't meant to
store anything, other than insulation in your lost.

Nah. I've seen loads of houses with 100mm loft joists. They're all used for
storage!

Floor joist sizes are so large to avoid flexing the ceiling as people move
about, not to stop the things collapsing. Light or even medium storage is
usually OK, as there is little movement. Any bad results are usually a few
hairline cracks, not stuff piling into the room below. Obvious, large water
tanks or big boxes of books need special consideration.

Christian.