Should I postpone my UCAT? Any advice appreciated

learn + review exactly where ur going wrong, make a bank of all q u get wrong coz ibr theres a lot of repition in patterns that come up - not just in ar but also stuff like qr and dm - as many syllogisms and questions as possible.
I'm sure you'll be fine, mock scores arent representative of the acc thing - ive heard and seen a lot of massive jumps ins scores. Basically walk in thinking ur gonna get 3600 lol and its gonna end well

Can you give it a lot of attention before the 31st?
Is there a particular section which you are finding particularly tricky?
Have a look at the main UCAT thread to see if others are wrestling with the same problems.
https://www.thestudentroom.co.uk/showthread.php?t=7475233
it depends on the date you can get to reschedule. If it’s the first / second week in Sept it might make all the difference. You have great stats otherwise.

I don't think it's wise to reschedule it to September as, you're right, you're going to be incredibly busy once year 13 starts. Do you really think it's worth pushing the date back and having a few extra weeks to revise? You know yourself best - if you genuinely think you will improve by then and it will make the difference between an offer and a rejection, go for it. If you think it's just a case of pre-exam jitters, it's better to get it over and done with.
Good luck!