Honestly, strongly don't recommend this if your aim is to go to uni.
Some unis and courses have issues if you do not do 3 A-levels in the same exam sitting (e.g. doing one early and the 2 the following year).
You also don't get "bonus points" for doing one early. And if you happen to be less well prepared than you think and get a worse result, this will be viewed just the same as if you got that result after doing over the full 2 years.
Additionally resitting in year 13 would still be seen as a resit which some unis and courses may have an issue with.
You basically gain nothing and risk a lot doing this. If you find it uninteresting see if you can swap subject or otherwise use the time you would have spent preparing to take it in one year intensively to do more wider reading or cocurricular activities that are of interest.