Well I live in the UK (I'm Canadian) and I think it's about as safe as anywhere else. I firmly believe that I'm more likely to be hurt or killed driving my kids to Canterbury for gymnastics later today than being hurt in a terrorist attack, and my DH and I lived in London back on 7/7/05 (I worked at Euston Station, close to St Pancras, Tavistock Sq and Russell Square).
We went to Paris twice last year (June, after Charlie Hebdo and December, so after the Nov 13 attacks) and found that we felt safe with the hyper vigilance. Same goes for Rome when I took my son there in January.
That said I'm a big believer in doing what makes YOU feel safe and comfortable. There are places in the world I absolutely would not go (as a solo woman, or with my DH, or as a family) but that others are happy to go visit. There's going out of your comfort zone, and there's genuinely feeling unsafe and not enjoying yourself because of it, but only you (and I mean the general "you") can make that call.