A long story short would be that the French government bottled it, Gladstone wasn't really enthusiastic but was talked into it by the French, the French backed down at the last minute due to political infighting and Britain was left holding the bag.
Poor Gladstone ended up having to take Egypt on as British even though it was something he was very much opposed to (as his letting the Transvaal go attested).
Intriguing - that would indicate that in TTL, with a different internal political situation in France, perhaps France would take the initiative, would not try very hard to get the Brits involved, and may take over Egypt alone?
However Gladstone policy comes out of a political situation where France is not perceived as a major threat. If France wins in 1870, the "dual control" of Egypt might be less friendly than in OTL (Gladstone might not even be in power, or his policy would be different by necessity). It might even end up in a situation where the British support the Khedive, and closer relations with the Porte, and France supports the Urabi insurrection: the first part of the war might be fought in Egypt by proxies, then who knows?