Signed up without knowing

I contacted O2 and explained that I was entitled to a government grant for satellite internet as we cannot get broadband in our area. I needed a dongle and data plan to cover me in the meantime. I was sent a link during an online chat by an o2 representative and purchased a data plan sim. I was sent the sim but as a signature was required and we work full time it took some time to actually receive the sim. No info was provided with the sim but in a recent online chat to try to set up an online account to view my data usage it became apparent that I had been signed up for a 12 month contract. If I cancel they have told me it will be £300 as I am out of my cooling off period. I am interested in the guidelines that a mobile provider should follow in making a customer aware of the contract and also the terms and conditions such as cancellation fee at the time you "sign up" to make the contract legally binding. 

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