So that's the £538 figure. It will depend on how sucky the 3G performance is compared to O2/Giff Gaff.
Running speedtest on my Lumia 800 I get around, 3,800 KB/s down and 1,600 KB/s up.
A work colleague's S3 was doing around 1,800 KB/s on T-Mobile and that's with an Orange mast about 800 ft away.
Odd, in newcastle my mate is on EE (t-mobile) and gets 18mb/sec (not 4g). On O2 i only get 1.5 up/down.
On three I get about 2.5 up and 4-6 down.
O2 is the worst network up here for some reason. And it was poor on the train down to London compare to Three / T-mob.
Orange / T-Mobile have the biggest network now, speed isn't everything if you can only get 3G coverage at home.