iPhone 4s has become locked after doing factory restore. Tried connecting to iTunes but still locked

HI my iPhone 4s was purchased unlocled from Apple. I have always used a Tesco mobile sim in it. I have just done a factory restore so that I can pass it to a friend who also uses Tesco mobile but the iPhone is now locked to a carrier. Spoke to Tesco mobile. They can't help as they didn't lock it. Spoke to Apple. They told me to connect to iTunes. Tried that and get the same message telling my carrier needs to unlock phone. Tried holding the home button down and connecting to iTunes. This forced a download of operating system but made no difference to phone. Went back to Apple who told me they could see phone was locked but we're not allowed to tell me who too. Apple gave told me to take it to a Genius Bar. Have tried to book an appointment at lakeside Thurrock and Westfield . No appointments. Anyone have any idea?