I spend anywhere from 2-4 months a year in London. I was there all of last summer when the acid attacks took place.
I wasn't worried. It's like hearing about a terrorist incident and automatically assuming the place is not safe. I was there for the Manchester bombing, the attack on the Westminster Bridge and the bombing at Parsons Green tube station. (In fact, I was supposed to pass through the Parsons Green station that day on my way to Richmond but had to cancel since that portion of the District Line was closed.)
You know what, I wasn't concerned. The chances of an attack happening in general, and happening where I am, is very slim. I'm more concerned visiting most major U.S cities than I am in London.
As Emma said, the perpetrators of last summer's acid attacks were arrested and are on their way to prison.
The best advice I can give you in regard to safety is....."Mind the Gap."