The Brits certainly had their work cut out for them. There were of course the usual concerns regarding Islamic terrorism, but in addition to that, they are always mindful of the sizable percentage of natural born British citizens these days who are leftist radicals. They run the gamut from Islamic sympathizers to environmental extremists. It would be a simple matter for them to mix with the crowd and wait for the most opportune time to work their mischief.
In fact, one of them was spotted fairly quickly. Here he is pictured below.
