I was asked on Twitter if I could find a good low cost iPhone 5, wasn't sure if they were after an unlocked version or one with a low up front fee to start and a low monthly tariff, I went for the low upfront cost and the low monthly fee and these 2 worked out the best:

Mobile Phones | Contract Phones | Best Mobile Phone Deals & SIMs

unlimited phone calls, unlimited texts and 1GB of Data for 27
Apple iPhone 5 16GB on O2 iPhone 27, 24 months

Mobile Phones | Contract Phones | Best Mobile Phone Deals & SIMs

On the same page:
Need more data? Here are some tariff recommendations for you...

unlimited data - that's right unlimited data!!!, 5000 texts and 1000 minutes call for 25 a month, 24 months

Both are web exclusives, this second option is 2 a month cheaper and can be found on the same page, just scroll down a bit: Mobile Phones | Contract Phones | Best Mobile Phone Deals & SIMs - It doesn't seem to have it's own page and you really have to go looking if you didn't know it was there