Funny you should ask that!
My youngest brother, like me an historian specializing in 20th century warefare, my father and I once walked the battlefields of the Great War. Found ordnance galore, but decided to bring back a full clip of .303 British bullets, plus a single one (that I kept) all in "as new condition" thanks to the Belgium mud. Just brought them back via the ferry, but always wondered what could have happened if they jad been spotted by customs!! They obviously were not the rusting shells or grenades that we found in bulk, as that would have been stupidly dangerous! But have never really researched what the law actually says on ordnance transportation, preferring to stay ignorant!
I guess as long as bullets are in good condition and you do not also carry an Enfield .303 rifle, that can be used to fire them, you are fine. A hand grenade with a pin that could fall apart / fall out, as we found plenty of, must be a different proposition however!!

But you say it is empty, so I cannot see that is a problem and would be classed as "deactivitated" or like a dummy grenade.
