Whether one is an enthusiastic European or not, the truth about Brexit from a national security perspective is that the cost to Britain would be low. Brexit would bring two potentially important security gains: the ability to dump the European Convention on Human Rights—remember the difficulty of extraditing the extremist Abu Hamza of the Finsbury Park Mosque—and, more importantly, greater control over immigration from the European Union.
One loss that would result from Brexit and which has been cited by Theresa May, the Home Secretary, as a reason for her supporting Britain’s continued membership of the EU, would be the European arrest warrant. But its importance has been exclusively criminal and few would notice its passing.