This is a bit of a non-story
First it's the daily mail reporting so I expect sensationalism and disregard things like "panic stricken staff" as the daily mail is full of bullshit like this.
Secondly the picutre is of a "similar" gun. In daily mail speak this probably means it's the first picture of a toy gun we could find and the actual gun was probably not similar in the slightest.
Thirdly evacuating the branch and waiting for trained police to come and assess the weapon is exactly what should happen. I really wouldn't want some random person who knows nothing about guns picking it up to see if it is really a gun or not, just because it looks like a toy doesn't mean it is, as the following picture shows, one of the guns in the picture is real:
if someone does pick it up then it really could go off and injure someone. Even if that doesn't happen it could still have been used in a crime and someone picking it up could result in evidence being lost.