Aren't they the people who advertise during the big bang theory? Never realised they were a mobile tariff company; good to know advertising just washes straight over me! ;)
Had a look on the site and doesn;t seem as cheap as I'd expected though. Can I transfer my number over? I've had it for 12/13 years now.
Number transfer: http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/How-do-I-keep-my-mobile-number/ta-p/3521127
The giffgaff deal is the cheapest rolling contract according to this: http://www.uswitch.com/mobiles/compare/sim_only_deals/# although it might not be suitable if you need lots of minutes.
If you do move to giffgaff, use an affiliate link from someone here and you'll both get a fiver of credit on your account. Does Velocio have one?