You can not retake UKCAT twice in one year.
UKCAT scores must be updated every year, if you reapply to a uni, and universities do not look negatively at this. Postponing your UKCAT for a later date is fine too, although I would advise you to inform the UKCAT team and rebook asap.
If you feel like postponing would improve your score and hence your chances, do it. Make sure your future date is available first though.
Best of luck.