Considering in 2010 (year from when the test is) the average was 17.7, your score is pretty good. Generally judging by the average scores since 2010, achieving 25+ should be a guaranteed win compared to the average, which has not been higher than 23/42. So as long as you arent scoring lower than that you should be fine. For essay writing, I asked 2 English teachers to look at an essay I wrote about a topic also from the official website. Maybe try that? Although timewise it might be a bit too late. Otherwise, read newspapers to stay updated with current affairs for the essay topic. I have my test on the 27th and am also applying to very tough unis haha let's hope it ends well. Good Luck!
(Also, incase by "high uni" you mean Oxford, the average in 2018 for accepted applicants was ~27, if I remember correctly, meaning I'm positive you'll be fine. At least regarding MCQs.)