But you can't change the country in your account once it's created. It's dumb. People move around, sometimes they even cross a border.
Ok let me clarify: when I created my PSN account, I set it to France. Now, I want to get a PS+ subscription. I (wrongly, being used to Xbox 360) assumed that as long as a code is european, I could use it. So I started looking for the cheapest option and that's cdkeys. They sell UK codes, which in the end are not compatible with my FR account, as you confirmed earlier. So either I have to get a French code if I buy a code somewhere else than PSN, or I just get the PS+ directly from the PSN at the regular price. I guess that I'm going to do the latter, it's just easier and faster.
I don't know if it is the same for France, but FNAC regularly has 10% off days which includes PSN Currency and plus cards so you can get a 50 card for 45. If you combine it with the typical 12+3 promotion, it is still a good discount.