Giffgaff don't have nano sims and neither do Tesco, I believe.
My family have been on giffgaff for a year or so now with various iphones and I am really happy with the coverage and value - however they can be a bit shambolic sometimes..
They are apparently expecting to have nano sims in the 'coming months'(!) which is causing quite a storm on their forums.
Their £10 250 mins UL text and internet is almost certainly changing to 750MB or £13 for UL soon, as well.
I intend to get my PAC as a safety measure (they send it to you automatically following a text request) and cut down / shave my sim when my i5 arrives on Fri - if I bugger the sim then I'm off to Three, with my PAC already in hand
I was going to order a Three 1 month sim and have it here ready too but the contract starts when the sim arrives NOT when you put it in your phone - if my cut-down giffgaff sim works ok I won't need the Three one and I can only imagine what cancelling is like through their call centre!