Worth pointing out that even though GiffGaff has the lowerst total cost of ownership, you can't get LTE on their (i.e. O2's) network yet.
I'm actually in the process of switching to 3's £12.90/month 1 month rolling contract[1] as they actually have the nano sims, though none of their plans have been included in the spreadsheet.
[0]http://community.giffgaff.com/t5/Contribute-Innovation-Promo...
[1]http://store.three.co.uk/view/searchTariff?deviceType=SIM_ON...
Which is a shame because between Voda (work phone), O2 (previous long term customer) and Three (past 2 years), Three wins hands down, previously O2 seemed the best - to me anyway.
Cheap as chips too, £15/mo PAYG, unlimited data, 300 mins, 3000 sms. Can reduce to £10/mo with a 12 month contract but that also caps the data. They're tethering friendly too.
As a side-note: I'm unsure about whether 3 will have their LTE supported as the article [0] is ambiguous about the 1800MHz spectrum EE sold to 3.
[0]http://thenextweb.com/apple/2012/09/12/apples-iphone-5-will-...