I bought my Prime card over the counter from the news agent for £3 cash as i also didnt have a card. I'm not sure they'll sell it to you if your under 18 but if you go with an adult they could buy it for you.

The thing is with these cards, if you want to reuse them to put more money on get access to a cash machine you need to upgrade on their website and i think you would need an adult to upgrade for you, though i know when i filled it out they did have a teen option so im pretty sure the card can be given to an under 18.

I suggest you check out their website fully first at idtprime.com or maybe call them.

Hope this helps