"I think to vote you should be able to demonstrated some sort of rudimentary knowledge of what you are voting for."
The problem with that is that there is always likely to be at least one issue on which everyone fails. My own knowledge of how economies work is so hopelessly basic that in a recent test to see where I was in the political spectrum (as a "D plot of economy + society), I came smack-bang on the middle for my economic stance. No surprises there, I had no idea what I agreed with ad what I don't. On the other hand, most people in this country would fail tests on, say, scientific issues, or at least the specifics, because most people have no real training beyond what they had to have at high school. Too, I can't really say I understand medical issues that well, but I have perhaps more experience of education from both sides than a number of people do. And so on.
Such a test, then, would almost certainly fail everybody if it tried to cover everything that politics is about. And politics is about everything, really. Further, there is likely to be a level at which such tests are opinion-based anyway. Should people do X or Y? Well, often the answer is that it depends, so how can you fail to pass that question no matter what you answer?
I'm against such a test, then, for two reasons:
-- Everyone would fail;
-- or only people with a certain set of political views, roughly speaking those aligned with the people who set the test, would pass;
In either case it would be useless. As much as I understand the frustration of having a vote cancelled by people you might regard as wrong, or those people who might have the classic opinion (as heard during the 2011 Riots) "It's the government's fault. The Conservatives, or whoever it is," ... designing a test to exclude them is almost certainly impracticable.
You would be better off, instead, trying to get more young people engaged in politics in the first place. What better way to change their opinions, or at least make those opinions more robust, than by encouraging them to get involved?