How to get acting credits?

As I explained in your other question, you have to get into a top drama school.
I don't know what you mean by 'drama credits'.
An A Level or BTec are okay for getting into a drama school - they show that you're already studied all the basic stuff. But that's just the beginning. Most students have to go out to work for a couple of years and save every penny just to afford drama school.
So as well as your college course you'll also need dance classes, singing lessons, and you'll have to perform with a good youth or community theatre to get loads of stage experience.
It takes a huge amount of effort to get the necessary training and experience to get into acting.
And yes - it is very expensive.

"Acting credits"? You need to be more specific. "Credits" as in part-qualifications in a university course?

What do you want to do with acting? Are you hoping to make a living at acting?