When choosing your A levels, you need to bare in mind that certain universities and certain degrees require different A levels.
So before reading, if you have a certain university in mind please check their site out and thier UCAS course.
As they may have A levels they prefer and A levels that they don't prefer.
Now in terms of an Economics degree you will find that many of the top universities require that you are required to do an A level in Mathematics, and to get a very good grade in it as well (e.g University College London specify that you are required to get an A* in the A Level).
It is also worth considering actually doing an A Level in Economics, although many of the universities say that it is not required, it certainly shows that you have an interest in the subject and see doing an Economics degree as a natural continuation of your academic education.
(If your college doesn't offer Economics as an A level, it might be worth considering to get this mentioned in your reference.)
Then personally I would consider doing an A Level in a science either Chemistry/ Physics/ Biology, firstly all the A Levels mentioned aren't going to hold you back for not being suitable to study Economics. But, they also add that aspect that you are able to comprehend the aspect of science.
(Make sure that you check the university if they have any preferences over the A level combination).
So now that you have Mathematics, Economics and a science, you may want to try and come across as a more diverse candidate, so you could opt to choose History/English Literature/Geography as this will show that you have a good ability to write an essay also. (Which to some Economics Degrees are essential).
You may want to also consider doing an A level in Further Maths to strengthen your quantitative side of your application further, as many of the top universities that offer economics do mention that this is considered a 'useful' addition. (But this A level can only usually act as a fourth A level, again check with the university.
Even if you are still unsure over certain options, email the instution.
If you are wanting to choose different A Levels to the following Economics, Maths, Physics/Chemistry/Biology, History/English Literature/ Geography and (Further Maths), then that is absolutelty fine.
All I have tried to do here was come up with a few options for those who aren't sure as to what subjects they should be choosing, I just hope you have done sufficent research into the A levels you are choosing and that they won't cause you implications in the future with your application.