When should I give the ESAT?

Hi. I am an A - Level student with 3A*s and 1A predicted grades. I want to apply for physics to the UK. I was planning to apply to both Imperial and Cambridge, for which I need to give the ESAT exam. However, I thought I could reappear for the exam in the second sitting in January for Imperial and do it much better as I would have learnt more content. Now, I have got to know that Imperial will only consider the first sitting of the exam. I don't feel prepared enough to give it now properly, but Cambridge only considers the October one. If I give it now, I may jeopardize the two courses I am applying to in Imperial. What should I do? Should I leave the Cambridge application and focus on Imperial, (I am confident I can crack it), or should I risk it for Cambridge?