I can't answer your original question "what would Keir Starmer do" as I don't know but....
If the Norwegians invaded UK, personally I would want "defence" rather than "attack". By which I mean, I would expect us to defend our own territory and people, but not attack another country and kill thousands of innocent people.
I would prefer discussion, compromise, and a political solution. But if that were impossible for whatever reason, then it would be OK to defend our land by intercepting missiles, waging war on invaders on our own territory, or using weapons to defend our cities and people/children/hospitals/significant industries/water supplies.
What I would NOT support is bombing someone else's land in revenge, incursion into their territory, taking over (or taking out) their industry or hospitals or destroying innocent people.
Unpopular view, I suspect, but this 'tit for tat' attitude in all warring countries doesn't solve anything and is only a dangerous version of the child's playground "you've bashed me up so I'll bash you up". Or even, "I'll bash you up first for no reason, just in case you decide to beat me up at a later date".
All war is bloody madness, and I hate it. I'm on no 'side' except that of the innocent sufferers. Each 'side' can cite equal, historically valid reasons for their actions but each are just as bad for not searching for peaceful solutions where each country lives their own lives without curtailing the lives of others.