I have just ordered a sim only plan, is there a time limit on how many payments you have to make before you can apply for a handset?
There's no time limit, It would be based on your credit score, depending on the result you might be able to order some or all of the BT range, it depends on the value of the handset.
When I took a SIM only with BT they told me that it was usually between 3-6 months. When I called up recently (3 months in) they said I could 'upgrade' immediately regardless of the time I'd had the SIM.
Am just weighing up BT vs Vodafone, my contract ends just as the new iPhone is out 😀