1. Economic Collapse of the financial system resulting in the breakdown of law and order. Imagine what would happen if all our banks and ATM's refused to give us money!
Our society would break down in a matter of weeks. 2. Terrorist attack on a massive scale - imagine what might happen if this country ever needed to declare war upon Pakistan or another Muslim country- we have nearly 3 million Pakistani's in the UK most of whom regard themselves first and formost as Muslims. Remember we only have 100,000 soldiers in the army and 125,000 police to deal with 3 Million plus insurgents.
On the balance of probability I believe the first scenario is most likely and may even happen within the next year or so...