Pay & go card

I have a mobile phone issued by my workplace. I can use it privately to a limited extent.Great!
I have been faced with an issue though, which is, I wanted to sign up to a site which required a minor amount on registration, but which has to be paid through a mobile phone - eg. send a text and you will be signed up. I don't want to abuse the privelige of the mobile phone I am supplied with in this manner.
Since, I've always been provided with work mobiles, I have limited knowledge of how subscriptions work. For example I had this idea I could get a 'pay as you go' card that I've seen you can buy in supermarkets. So, I was thinking,if I want to sign up to this site, can I buy such a 'pay as you go' subscription and use this? Will such a subscription provide me with a Sim card that I can then insert in my mobile and use to sign up for the site, and then afterwards I can reinsert my original sim card back in? Any other ideas? I was thinking of asking somebody else if I could borrow their phone and just pay them cash, but since the site I wish to sign up for is a dating site, it could be a bit akward, he he. Especially if the friend is married..;-)
However, I really don't want to start a subscription for such a one off show either
Thanks to all who respond