CPA NETWORK
Here's how you would go about it.
1. Approach a big company like Neverblue or AzoogleAds.
2. Specify that only people in the UK can do this offer (easy to do, widely done)
3. Simply do it in the same fashion as a Mobile Offer, or do some king of thing where they can make money if they download from the website. You have to be creative here and come up with either a mobile or lead offer where the affiliates are happy and you offer them a great payout and the customer has an incentive to do one of the two actions you mentioned (either sending the SMS or Downloading your song).
Now here's the thing, unlike a normal Mobile offer that's hard to get to convert because people have to send a special text to knowingly subscribe to a $10/month service, yours is way simpler and it seems like you have the money to do it.
I propose that you give people some kind of incentive, like a free download of the song, or whatever you can offer and in order to receive the gift the user has to send your specific text to some service. Say it's really easy and they won't be subscribed to anything, explain it to them and how their is no risk involved since their just sending one sms. Now that's the from the point of view from the traffic created by the publishers of the network you put this offer on.
In terms of taking care of the published and giving them a reason to promote you're offer, I'd offer them something like $3.50 a lead. It's great for them because it's easy to promote and the user barely has to do anything. You'd have to figure out how to track it but maybe you could send it through a middle-man type number before it goes to the destination. I'm no pro at sms though...
Hope that gave you some ideas. If you have the money to spend (seems like you'll quadruple your expense regardless), I would definitely go with a big network and their 1000's of publishers instead of trying to do it yourself.