Perennial....not if frosted.
It is normal in cold areas of UK to dig the tubers up and dry them and store in a frost free, cool dark place over the winter. Then start them into growth again in the spring.
Yes, looks like a dahlia. It would be perennial but, depending on climate, the tubers may need to be stored somewhere for the winter. As far as I remember whether that is necessary varies depending where in the UK.
