“I feel for the relatives of dead British soldiers who want to attend, but Australian casualties in this battle were far more significant numerically”
Hmm... During the Gallipoli campaign the British suffered 74,000 casualties, the Australians ‘only’ suffered 21,000. By this logic, the Australians should have been excluded from the Gallipoli commemorations because of the relatively few casualties they suffered.
If we were talking about entire Battles (in this case the Battle of the Somme, rather than the smaller Battle of Fromelle which was part of that overall campaiign), yes. But we're not - we're talking about specific locations in those battles of particular significance to particular nations. Australian commemorations at Gallipolli centre around ANZAC Cove, they do not incorporate the entire area of Battle - and they settle around ANZAC Cove and Lone Pine because that is the area where Australian and New Zealand troops were the dominant presence and the dominant casualties.
British commemorations at Gallipoli generally take place at the Helles Memorial. Britain regards it as a relatively minor campaign in its own history, so their commemorations tend to be much smaller than those of Australia or New Zealand where it is considered one of the most significant events in our history.
Likewise, to the United Kingdom, the Battle of Fromelles is a minor footnote in its overall history. To Australia, it is our bloodiest single day in war - sometimes referred to as the worst day in Australian history.
Divisions by area like this across the larger battle sites to allocate particular areas to particular nations in terms of memorials are commonplace and have been throughout the twentieth century.