Rather more faff than unlocking an Orange San Francisco, but apparently you can do it yourself for free these days: http://android.modaco.com/topic/348403-want-to-unlock-your-omc-skate-tutorial-here /
(Compare http://sanfranciscoandroid.co.uk/android/how-to-unlock-the-orange-san-francisco-zte-blade-for-free / But then the Monte Carlo is newer, bigger, faster. My SF was effectively only £60 because I bought it with Tesco vouchers while they had a 2-for-1 offer on. The iPhone is a much nicer phone of course, but so it should be at that price.
GiffGaff is £10 a month for "unlimited" net. (Where "unlimited" means "fairly unlimited so long as you don't take the piss": http://www.theregister.co.uk/2012/01/11/giffgaff_unlimited /)