I have a ridgeback velocity which is labled as double butted aluminium. Around the middle of the toptube is a dent shown in the photo. There is also a short line above it which I can feel with my nail. Would it be safe to ride?
Yes it's safe to ride. That "line" it's just a crack on the paint surface, I would suggest painting it with something like clear nail poslish jusft in case. A dent on a commuter bike is nothing to worry about.
the dent looks small enough that it does not pose a safety risk, although a picture with something included for scale would help confirm that.