Mers-el-Kebir was brutal but strategically understandable

How does this assessment justify the British attack at Mers-el-Kebir?

He says the attack was justified from Britain's strategic position, while also noting that Somerville disliked it and that French command communication made the crisis worse.

In this assessment, Britain could not easily accept the risk of a large French naval force becoming available to Germany. The British likely had contingency plans for many harbors because serious navies prepare for unlikely but dangerous cases. Somerville sent a wider range of options than the French admiral at Mers-el-Kebir apparently received through his command channels. That communication failure worsened an already grim situation.