Having been a QA for 5 years now I can tell you that you don't need BCS/ISTQB at all - in fact it is seen, by a lot of testers I speak to, as a bit redundant and out of date now
Best advice to you would be to sit with the Dev's (if you get chance) and look at them creating the next version of the app that you will have to test, or a bug which will have to be tested. Learning how things hook together and how things are created will help you understand how stuff could go wrong, and that will benefit your testing
Where about's do you live? Here in Manchester there are lots of 'Tester Gatherings' via Meetup where QA's from all over gather together to talk about stuff etc. If you can get along to one of those and speak to people doing the job you will get a better idea of which direction you want to head in.
Also, I 'fell' into testing. No qualifications, nothing on my CV regarding IT or anything like that. The company I worked for at the time needed someone who had lots of knowledge of the company (I was a Staff Training Manager at the time) to look at a web site and compare it to the companies own. I spent a few days pulling this site apart and after that the IT Director wanted me to work with the other Tester in the company and offered me a job.
EDIT:
I would say you have a good start to become a general tester for mobile applications. The key thing really is to understand who the target audience is for the application and the company which the app is for.
I went from working as a tester for a car hire broker to working as a QA for a hotel booking web site - both are travel and both were for the general populations consumption. Knowing the industry and understanding the customer and some of the crazy stuff clients actually do, rather than what they are supposed to do, makes for a great tester.