That is already a possibility... for example see the Iranian embassy siege, for example.
Likewise at times in Northern Ireland it was deemed preferable to wait for situations where IRA members could be legally killed rather than pre-emptively arrest them.
I'm not talking about the shady allegations of collusion but say receiving intelligence that an attack is to take place and rather than arresting the members of an IRA unit and then not necessarily having sufficient evidence to necessarily convict all of them instead take the risk, wait for them to carry on and attempt the attack and then ambush and kill them.
The UK isn't a war zone and that sort of comparison is wholly inappropriate, we're talking about a system of justice here in peacetime.