If you have a provisional licence and passed CBT (compulsory basic training) certificate. Then, according to current regulations in UK (https://www.gov.uk/ride-motorcycle-moped/bike-categories-ages-and-licence-requirements) you can ride motorycles with a power output not more than 15 kW.
Zero FX and Zero FXS have the smallest power among other electric motorcycles, it is only 27 hp or 20 kW.
Therefore, unfortunatelly, currently you are not allowed to ride electric motorcycles with CBT certificate. You have to pass full exam. Although it would be much easier with electric motorcycle, because it is much easier to control electric motorcycle then a petrol one.