The most important ingredient to a high UAI is to ace all your subjects. While we place a huge emphasis on English these days (being the only compulsory subject in the NSW HSC), the hype over English is slightly overrated by now. Put in a simple way, if you ace your HSC English but ace nothing else, your UAI will be mediocre at best. Instead, do not over-focus on any particular subject. Every student's goal is to maximise their UAI, so their best ten units count towards their all-important UAI. The obvious thing here is that each unit is equally weighted, and there is no inherent reason why a student should treat HSC English any different to any other HSC subject they are doing as part of their 10 or more units.
In fact, the rational way to allocate study time is to allocate more time to the higher-scaled subjects, as they will be more beneficial to your UAI than lower-scaled subjects. Historically English Advanced has a scaled mean of around 30, which is practically equal to the scaling of HSC Chemistry and Economics in the past several years. These trends are not expected to change significantly year to year, so we can expect the same for at least the near future. If you're a student who does English Advanced and Maths Extension 2, you should definitely focus more of your time on your mathematics subjects, given that both Maths Extension 1 and Extension 2 counts for 2 units each. Keeping in mind that the scaled mean for Extension 1 is around 39-40 in recent years, and that of Extension 2 is around 43-44, this makes these subjects unparalleled in terms of scaling power, and definitely deserves more attention than English.
That is not to say you should neglect English either. Treat it as you would any other subject. There is nothing special about English, except for the fact that it is the only compulsory subject in the NSW HSC, because of the way scaling works and UAI is calculated, which requires a common denominator for comparison. However this mere fact does not affect the individual student. There are rumours and misinformation floating around over student forums and by word of mouth, but the simple fact of the matter is that English is just like any other subject and is given no preferential treatment when it comes to UAI calculation. Therefore if this is the case, English should not be given any special treatment by students either. What this means is you should split your time equally as you would any other subject that scales similarly (e.g. Chemistry, Economics).