Hi there!
Choosing between different engineering courses is much easier than between 2 vastly different courses so the course content does tend to be similar in certain areas. The general idea is that all of the courses focus on your maths and physics skills, so now is the right time to start focusing on developing your basics in those and ensure you're comfortable and happy with it. Also, it's a good idea to look through what the course modules for all of the courses are, since they can give you a very good idea as to the type of work you'd be doing on it.
As for the course breakdown itself:
Aerospace engineering -
Pros - You'll be doing a lot of simulation and analysis work especially further into the degree, which can give you a very good head start on learning industry software
Cons - It's quite specific since it's just aero so you could find yourself lacking in certain academic areas (eg. mechanics and dynamics), you will also find yourself not using a lot of the curriculum (this is mostly about things more aimed at the aircraft industry) if you are aiming to head into the motorsport/automotive sector. Pretty much you need to know now if you want to only stick to aero roles wherever you go.
Automotive engineering -
Pros - It will teach you a lot about all of the different aspects of vehicle dynamics and performance, the modules cover a lot of ground from an engineering aspect so you do get to learn about a wide range of topics.
Cons - Also very specific like the previous option, you're focusing solely on analyzing different areas of a car for example. While you do learn about a wide range of different engineering principles, you are only really learning about them in the context of a vehicle so it can be harder to then apply to other scenarios, which might be either similar or differ in the actual world.
Motorsport engineering -
I won't do a full breakdown of this one because it is genuinely very similar to automotive, but instead of just looking at a variety of vehicles (cars, vans, trucks etc.) it is very much geared towards motorsport vehicles instead. Fewer universities offer this course so you would be limiting yourself in where you can apply to.
I know that I've said for all of these that they're quite specific, but it's honestly just the truth. When you're choosing what type of engineering you want to do, it's not that you're necessarily blocking off various career path options in the industry, you just might find that you're lacking in certain areas when in comparison to your peers, depending on what they did too. Because of how specific the three industries associated with what you're choosing between are, a lot of the more specific information (and/or software) is learnt on a job rather than something you know going into it.
Not to also throw new ideas at you, but if you're deliberating between several specific engineering courses, Mechanical Engineering could also be a good choice. The course covers a wide range of topics, which gives you knowledge in a lot of different areas and it tends to also be a quite popular course. You can also specialize what field of engineering you want to get into once you get to the Masters Level and have had time to explore different areas before you choose what you want to do specifically.
It's not that it's technically favoured in the industry to do mechanical, but you do tend to find mechanical engineers in a whole variety of roles due to their versatility and understanding of a lot of those key engineering principles from a range of different areas.
All that I've said does however depend very heavily on what sort of career you'd like to go into in the future. If you're looking at Aerodynamicist roles, then Aerospace will be your best choice since the course covers the most on that area. If you looking more at other engineering roles, then the 2 other courses are a good choice (not saying that Aerospace isn't good for it, it's just that the course is geared much more heavily towards the aero field).
The main thing I'd focus on in your case in finding the course where you feel you'll be the happiest, because a lot of the knowledge needed for this sectors comes from experience once you get into the workplace, so don't feel like you have to make a choice purely based on the role you think you want to do now before you've even started an engineering course.
Finally, as I tend to say for anyone going onto an engineering course, it is highly recommended that you join the Formula Student team (if the university you apply to has one). It is a competition run by the IMechE, the Institution for Mechanical Engineers and the accreting board for mechanical engineering. It is internationally recognized by employers and the best place to put your learning to use in a practical environment, plus it's just super fun to be a part of and there's a really strong community aspect to it.
If you do have any questions, at all, that you'd like to ask, feel free to and I'll respond to them. I am a Mechanical Engineering student personally, but a lot of the engineering disciplines and content does tend to overlap so I can respond about any one of the degrees you are looking at doing.
Wishing you all the best,
Ams - LJMU Rep