Train up more armed police.
Bring in the army, if any extra support would be necessary.
I'm sorry to say, but this is war. War not started by us but started by jihadists. The West can either lie down and take it up the *rse from terrorists, or it can hit them and hit them f**king hard.
The interwebs allows jihadist groups to inspire a variety of home-grown malcontents, sexually-inadequate tossers, and other f**kwits. They should be shot dead on the street as and when they attempt acts of terrorist numb-skullery.
The Stop the War Coalition and other naval-gazing, self-loathing oxygen thieves can f**k off.
I hope the injured make a swift recovery. I hope the terrorist loser gets raped in prison.