Go here for the premium thingy:
https://p.nintendo.net/premium/login.do You get eight points per £1 spent, meaning you'll be eligible for your £5 voucher after spending £62.50 because you can only cash in after 500 points.
You can just spend the exact amount for the games. Instead of topping up, just click "pay remaining balance by card" and it will just charge you the exact price of the game.