There might well have been a noticeable increase in the number of
emergency services vehicles that you will see going to incidents on
blue lights. The number of security alerts has gone up very
substantially since the bombings -- people aren't willing to give
unattended packages the benefit of the doubt (and quite rightly so).

The problem is that these alerts need a lot of people to deal with
them, especially when it comes to clearing people from streets and
buildings.

This, coupled with people being very aware of the emergency services
around them, probably explains why people think there are more police
around than there were before the attacks.