.My thought is that the 100mm as for railings etc applies but this seems too large to comfortably walk on.
Do you really mean minimum (or maximum)?
yes maximum spacing would be more accurate
puzzled by that last comment…did you mean to post this for another question?
Apologises for the delay in replying. As you have noted there are no real controls as such for floor grills within the building regulations (unless it forms part of a staircase or platform), but there are controls under british standards for floor grills (spacing may be up-to 25mm for gaps) and there are also standrads for checker plate design in industrial situations where gaps of up-to 100mm*50mm is permitted, but this is for industrial situations. I would be wary of using 100mm spacing in domestic situations where children could easily trap or hurt themselves. the orginal building owner and also yourselves as designer risk being liable for injuries in this situation.