Speak your heart, nothing beats original.
What I mean by that is, write about why you did not choose Economics at A-Level, it is ok to say what you said earlier i.e. you were not sure but then make sure you write a strong paragraph about why you have now chosen to take it up at degree level.
It is ok to not be sure, you are only young and cannot be expected to have a planned life. Even the grown up haven't got it all figured out.
Coming back to the point If you have A-Level Maths, which you do appear to have then you should be good. First year is a recap of A-Levels anyway. Just study hard in the first year and you will be set-up for success.
All the best
Sumit