Can you clarify what you mean? You state SIM only, but then you say you want the contract to come with the phone.
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Ah, I think I've deciphered your post. It was confusing as you've listed the cost of the phone. If you are going SIM only that cost is irrelevant. (Or at least up to Tim Cook not you!)
Do you have to have 4G? If not, then Giffgaff are the cheapest (and technically run over O2.) Only hitch with Giffgaff, not a supported provider so no Vis Voicemail and no tethering without a jailbreak (even though tethering is allowed on some of the deals!).
I use Giffgaff - and the money they save me every month has contributed nicely to pay for this iPhone 5s. 🙂
But I am waivering whether to stay with them, due to their lack of 4G and tethering. Both likely fixed in 6 months time (when jailbreak for i5s is released and Giffgaff launch their 4g packages.)
One other thing I'd think about. I was sorely tempted by 3, so I grabbed a PAYG SIM from them to try them out. I found out that their reception where I live and work is terrible! We are a landline-free household, so that was a deal breaker for me. If you are going SIM-only, you can try before you buy, as long as the network offers a free PAYG SIM (you can't make calls without topping up, but you can test signal strength - :-D)