I can only agree with everyone who's against compulsory voting. I see two main problems:
1. A waste of time and effort on many people. So many will go and return "none" or invalid ballots. So, what's the point in forcing them to do that?
2. A result much less representative of what the people vote. You force people to vote, and you're guaranteed that a good portion of those who would've abstained will vote for someone semi-randomly. Because they like the number the party's listed under, because they think a candidate's name is cool, because he looks good, etc. You already get some of these voters, but with compulsory voting, there will be many more such voters.
So as a result, compulsory voting would seriously undermine democracy.
Compulsory voting would also probably be hard to introduce in most countries. For example, my country's constitution explicitly mentions a "right to vote", which, not being an "obligation to vote" means you can abstain. And changing the constitution is quite hard. Then again, the UK, who proposes this time, has no (codified) constitution.