Why did Mycroft involve Sherlock in S02E01 when he already knew what was happening?

I don't understand what investigation was meant to be done by Sherlock since he already knew about the terrorist attack and had a remedy for that?

Mycroft involved Sherlock to obtain the photos of the compromised royal person from Irene not in relation to the terrorist plot.

Sherlock only got involved with the Coventry plan because he had the phone and was showing off to Irene.

He wasn't supposed to expose the plot, Mycroft didn't involve him in that at all, let alone ask him to investigate it.

MYCROFT: This entire project is cancelled. The terrorist cells have been informed that we know about the bomb. We can’t fool them now. We’ve lost everything. One fragment of one email, and months and years of planning finished.

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