//Mozz apologies you may have further observations here but I'm answering as I would had I suggested the net contributor idea.//
No apologies needed TTT, I thought things like an age limit were an obvious given, but I should've known better. Let me put it this way: if you have left full time education, and have gone into the workplace, you can vote. If you have been in the workplace previously, and find yourself currently on benefits or retired, you can vote. If you are a homemaker you can vote. If you are registered disabled or unfit for work, you can vote.
If you are in full time education and have never worked, or are sat on the dole since leaving education, no vote for you. Nor do you get a vote if you're voluntarily living outside of the UK.
I'm sure there are tweaks that can be made for different scenarios, but that is what I consider contributing to the country.