No 100 I is No. 100 Mk 1 Percussion Fuze which was used on a variety of shells including the 18 Pounder High Explosive Shell. The 18 Pdr shell is 83mm / 84mm in diameter.
The shell you have is unfired, the driving band (Copper band) has not been engraved by the rifling. I have not seen a base like that one before on 18 Pdr shells. I would have to get into the books to positively ID it.
There is no way to tell if it is actually inert from a visual inspection of the exterior.
In the second last photo, the marks look like the tip to tip broadarrow sale marks. If so, these, along with the 1914 WAR mark could indicate that it was a curio.
But if you cannot confirm that the shell and fuse are inert, don't assume that they are.
Might be a good idea to find out if it's inert or not. Would be interesting to see how large the evacuation perimeter would be while they check it out.