honestly, i doubt your school would allow you to change at the very end of the year. physics is also extremely difficult and it would probably be hard to catch up/teach yourself a whole years worth of content before year 13. i do feel your pain with the no teachers thing, i had to have replacement teachers in all 3 of my subjects - including psychology. what i'd suggest doing is spending a portion of the summer holidays going back over year 1 content and potentially getting a head start on year 13 stuff using youtube videos and textbooks while you wait for a replacement. other than that there's not much you can do except for maybe resitting the year and changing your subjects.
its your sixth form/colleges obligation to find you a supply, its your a-levels... they cant just leave whole classes without someone to teach. i'm sure there have been complaints already but make sure to be annoying and remind your school/ask about a new teacher at every chance possible.