Fortunately, if that word can be used about anything today, London is blessed with excellent emergency services who are experienced in dealing with events of this type, and who had well-developed plans for responding to the attacks. By all accounts, they have been doing a great job.
I was in the centre of Glasgow about 2 hours ago, and the police there were out in force, spread around the city keeping watch. I expect that similar scenes can be found in Birmingham, Manchester, Edinburgh, and other key cities. It is very reassuring that someone has gone to the effort of putting response plans in place, and that this wasn't the surprise it might have been. (It was, of course, no less of a shock for all that.)