Mercenary gets it.
What is the point of voting/voting in a safe seat? Making the sitting MP feel mortal, creating a virtuous network effect and clawing away at their majority. That is, there exists an opposition not just somewhere out there, protesting in big cities, but close to home and it is active - that wakes MPs up. It also signals to other like-minded voters that you exist and agree with them. Thus improving morale and turnout. Furthermore, demonstrating active constituency opposition is more effective than people think - it's personal and psychological. Then the sitting MP also won't be able to count 'the silent' minority in their mandate, which they can if you just don't turn up and rail online. Protest without votes, or petitions without both, have a very poor record of achievement.
Put simply: register, turn up, vote; and be vocal about it. Indeed, matching the regular turnout of 60+ y.o. voters with a <40 y.o. turnout equivalent, even without an explicit guarantee that they vote Labour/LD/etc, would still produce a more moderate result, and can defeat a Tory majority/force a coalition - every time.