Have you any paperwork which said it would come registered in the UK? If you do, and it didn't, you can reject the item using the sales of goods act. It doesn't matter about the 5 minute warranty they may have offered, it has to be "as described". You can register it yourself and the process will depend on what paperwork you have and how old the bike is. Don't forget to say hello in the new members section as people who do generally get better replies to their posts.