It's worse than that - trespass isn't a criminal offence in the UK (http://en.wikipedia.org/wiki/Trespass), it's a civil matter, and so punishment doesn't even come into the equation.

"it doesn't work as a protest"

It absolutely works as a protest. The reason governments are able to govern in so-called democracies is because of a thing called mandate ("an authorization given by a political electorate to its representative" - The Free Dictionary).

If you vote, no matter who you vote for, you are giving the elected government a mandate. If you withold your vote, you deny the government a greater mandate. At some point (don't ask me at what point that would be), when the turnout for an election is so low, the elected government loses legitimacy - only then will we stand a chance of seeing some real change and real democracy.