You're welcome, glad it was a helpful answer
To be honest I really don't think you need to have studied a Game BTEC to be prepared to do a Game uni course, or even to create a good portfolio
The real questions would be whether you think you will be ready to move on to uni straight after A-Levels, and whether you would have the self-discipline to create a portfolio in your own spare time independent of school/college? The first one probably seems like a redundant question but I actually know a lot of people who weren't ready to start uni at 18 and waited a few years either working or doing extra courses before they started uni, so if you think doing a BTEC in that sense will help you then go for it. The Game Development BTEC I studied was good and the content was definitely relevant to what I'm doing now, but I don't think the people on my course who did A Levels instead of BTEC are at any kind of disadvantage. The only things I (and other BTEC game students) knew that the A Level students didn't was a basic knowledge of programming and a better understanding of the game industry, but both of these could be looked up in your spare time.
As for the portfolio, it's definitely very important, but I think portfolio pieces that have been created independently rather than as part of a college course definitely have more weight with admission tutors. Plus a lot of the stuff you are specifically interested in might not be covered in the BTEC course.
That said, if there is any shadow of doubt in your mind as to whether you really want to go into game design, I would 100% recommend doing the BTEC course to really get an idea of what it's like - I'm assuming you are still at the age where college is free and only costs time. Trust me, if you are unsure at all don't spend £9k for a year at uni only to realize that you don't actually want to go into games, if you have doubts do the BTEC to work out if it's what you really want to do.
If you want to start creating things for your portfolio (I really recommend starting this summer! It's amazing how long completing a good project for your portfolio takes, and having the pieces of work to show off are also useful for your career not just uni) here are a few free pieces of software I have used for my work, at uni, college and in my spare time:
https://www.yoyogames.com/studio - Gamemaker, a very basic piece of game making software. Basically no programming if you want to avoid it.
http://www.stencyl.com/ - Stencyl is also a pretty simple piece of game making software but more complex and better that Gamemaker in my opinion. Makes you think about programming theoretically but again you don't actually have to use programming if you don't want to.
https://unity3d.com/ - I've only just started using Unity so don't really have that much to say about it. It is a much more sophisticated and complex piece of software and is an industry standard game engine. You will have to use some C# if you want to make things with Unity, but nothing too heavy going, and it is worth it.
http://www.blender.org/ - Blender is a really good free 3D engine. Handy for teaching yourself how to make 3D models and animations.
You might also want to read up a bit on different job roles in the industry, paper prototyping and level design/design documents. And if you don't already play some indie games to get an idea of the sort of things you could be capable of making!
This website
http://www.digra.org/ is handy too because it has an archive of games research papers and write-ups from talks and conferences. There is a big range of papers so some of them are pretty dry and heavy going, but if you can read a few of the lighter ones it will give you an idea of what the game studies aspect of your course will be like, and what the academic field of games is like!
And in general I recommend creating a blog or a website to present your portfolio on, it just looks good and later on you can use it for networking with other game students and devs
http://wordpress.com/ is my preferred site for that kind of thing.
Edit: also if you want to give yourself a crash course in programming
http://www.codecademy.com/ is a good place to start.