I personally feel the fees are the killer however it is a viable idea.
Perhaps an alternative model could help, I'll chuck out my top of my head ideas:

• Sponsored till rolls
• Higher upfront fee
• lower fees the more you use (send under £50 pcm fee =3%, spend over £50, fee lowers to 2% etc)
• Obtain a referral deal with an exchange

Just some ideas, good luck. I would like see this.