@SerendipityJane However I would (and just have 😀) suggest the way to counter this is not to resort to mandatory voting, but just to require a winner to get more than 50% of the available votes.
You can bet your life that if Reform or the Communist party actually had to work to court 50% of the electorate, you'd get much more moderate and accessibly policies.
Assuming that you mean 50% of votes in a constituency, then there are very few constituencies where the winning candidate gains over 50% of the votes cast.
Your model would actually result in an exaggeration of the current situation - there would only be an option of voting for one of two parties, since no other party would stand any chance of winning. In a 3-party constituency (e.g. where Lab, Con, and LibDem all had a similar percentage of support) how would you allocate the seat?
How does this resolve the issue of governments being formed based on the votes of around 30% of the electorate?
How would you ensure that the views of the minorities (which might be as many as 49.99% of the electorate) are taken into account?
Why should people be forced to vote if the only choice is between "Dumb and Dumber" - which in many constituencies is already the current situation? Why should they be forced into endorsing one or other party of which they don't approve?
How will the country ever change from the hegemony of two established but out-of-touch parties if the barries to change are even greater than they are today?