The wormers you can buy in the pet shop aren't as strong as the ones that the vet sells. I suppose it's a bit like the paracetamol you can buy at the corner shop and the pain killers the doc writes out a script for or the pharmasist can sell you.
You can now get Drontal wormers from Pets at Home - don't know how much they are just noticed the big sign outside it when I was going past the other day.