It seems that Samsung offered free Galaxy S devices to users who declared themselves unsatisfied by Apple's iPhone 4. The only thing that these users from the UK to which Samsung sent these devices did, was to complain on Twitter about the iPhone 4, so that Samsung contacted them offering free Galaxy S devices.
It seems that this is a tactical move made by Samsung, which tries to make a little advertising to its devices in order to increase its market-share. Of course that these devices were sent to people who are really influential, so if you are a normal Twitter user, don't even mind trying to receive a free Galaxy S. You can try if you really want, but there is almost no chance of getting one. And don't forget, devices are being sent only to important people from the UK.