when should I register for the UKCAT 2017 and when is the next exam for 2017?
Registration opens in May. Its not like GAMSAT where theres only a few tests. You can book into a huge number of test centres around the UK on a date that suits you. If you book early most dates are available within the testing period (May-Sep).
Most people recommend you book for sometime in July-August as this gives you adequate time to revise. Although obviously, everyone is different so book whichever date and time suits you!