I hate to jinx it, but how come we haven't had something like this yet? Are our security and intelligence forces too good?
There is the obvious fact that being an island, it works in our favour to make smuggling weapons into the country difficult. But ultimately, the 7/7 attacks showed us that people intent to kill don't need guns.
If this is ISIS related, one of the key things that can be said is that Britain aren't currently involved in air strikes against them. That's not to say that we aren't already a target, regardless.