If you've now conceded that, under the current legal definition, Southport was not terrorism, then we can move on to whether the definition of terrorism should be broadened, including the current requirement for the act to be in a political, religious, racial or ideological cause.
It's perhaps surprising that more of the people "protesting" after the Southport murders weren't charged with terrorism offences, as their actions could be seen as acting more "in a cause" than the murderer (under the existing definition).