Millions of people live in London and take the tube every day. The chances of anything happening are very small and if something does happen, the chances that you will be in the wrong place at the wrong time are even smaller. It's like the lottery - someone always wins, but the odds are very long.
There are frequently posts on here from people who don't live in London saying they wouldn't go there now, post the Paris bombings. But of course nothing has happened in London since Paris and those of us who live here have continued to get the tube, get buses, go to crowded places including theatres and concert venues, go to airports, get trains at major London termini....
Something bad probably will happen at some time, but if, like me, you have lived in London for most of your life you realise that the chances of being caught up in a terrorist incident for any individual are vanishingly small. As previous posters have said, you are much more likely to be involved in a car crash. We are not very good at assessing risk and this makes our judgment flawed.
As my teenage son said after the Paris bombings, you've just got to carry on living your life and doing what you want because you can't control what random bad things happen. No one expects to lose a leg when they go to a theme park. No one expects their child to be blown off a bouncy castle and killed. No one expects a sofa to fall on them from the top of a building. And we don't spend much time worrying about those things, all of which have happened in the last year. So our approach to terrorist threats should be the same.