On the PC: "This Prepaid Card cannot be redeemed on a PC." (normal for a valid PS+ code)
On the console: "The code you entered may not be correct or may no longer be valid. Please check your entries." (I did double-check several times since I can't copy-paste from the forum)
So that's odd.
No, when code is used will appear on PC: "Code already redeemed". If shows: "Card cannot be redeemed on PC" is because it wasn't used yet. And for last: "Code not correct no long valid" probably means region of code/account don't match or you had used a trial already or suscribed Plus on account.You're an hour late in posting, so it's most likely used.
So, check if your acc is UK and if you never subscribed or used a Plus trial on it. And you have to redeem it on your PS3 system, not PC. You will need a credit card info either.