[beachboy]

"I wish I bought a car last week in London" is wrong. You'd have to say "I wish I had bought a car", meaning that you didn't buy it, or even "I wish I could have bought a car", meaning you couldn't buy it. If I could have a guess, I'd say that the difference between "I wish I had a car" and "I wish I could buy a car" is that to have is a verb of state, and to buy is a verb of action, but it's not clear for me.