I disagree with you slightly there Col. I think the UK's membership of the EU is not the biggest threat to your security. I think it is your membership of NATO and your involvement in the Iraq war which is what triggered the current unrest in Iraq and Syria. The UK and other countries who participated in that war have been subjected to terrorist attacks also, France and Spain come to mind. Lots of other EU countries who didn't participate in the Iraq war have not been subjected to terrorist attacks, ourselves for example. The jihadists you mention want to enter Britain to carry out more attacks because of the invasion of Iraq not because of your membership of the EU.
At no point did I go into the reasons behind the terror attacks.