The question of the moment on 23 June 1942 was where to fight to stop Rommel. The Defence Committee in London would have liked to see a stand made at the Frontier. For a number of reasons, this was appealing. First, it would keep Axis forces further away from Alexandria and the Nile Delta. Secondly, Allied Air Forces would be better able to protect convoys to Malta and to interdict Axis supply lines.
In the Middle East, the commanders thought that they lacked sufficient mobile forces, especially armour, to risk a fight at the frontier. Mersa Matruh was another 120 miles East of the Frontier. They calculated that there would be that much more strain on Axis supply lines by fighting at Matruh. The problem with fighting at Mersa Matruh was that there still was a lack of mobile forces and defending Mersa Matruh without them was problematic. This is based on the account in Vol.III of the Official History.