When i first stepped into the world of pay monthly, i got a sim only contract with o2 on a 30 day contract. I used this for 3 months and then upgraded to a full pay monthly contract with a free phone.
I think doing it that way helped me as i was a minimal risk to the network as i didn't have a phone from them(just a sim), i could have got out of it at any time, but i stayed with them, and i paid in full every months, which helped prove i am a good customer.