Look mate, on a human level yes it certainly is NOT insignificant. Anybody that is killed is a great tragedy.....that being said, in terms of crime stats, terrorism is absolutely insignificant in the UK.
Now, with regards what I think we actually should be worried about, I'd go with: The disparity between funding for mental vs physical health...you are several hundred times more likely to kill yourself than you are to be killed by a terrorist. Heart disease, alzheimers, drug addiction, the government spying on it's citizens (mostly for no good reason) are all issues which affect society WAY more than terrorism.
Will terrrorism become much worse in the future in the UK? Maybe, though the general trends in terrorism committed doesn't tend to support that view.