Salams.
The January sessions are usually for the re-sits or the people who give half their AS or A2 Levels; you cannot give all the modules available. So, either way, you are going to have to sit the June session.
So, I think it is best to sit all the papers in the June session. Also if you give your exams divided in two sessions i.e. Jan and June, some universities (especially American and British) may look down at these candidates, as often the Universities want people with just one sitting and not multiple sittings.
And taking econ would be an excellent idea, because this way, you still have the financial and science side open. Furthermore, it completes your 4th subject, and gives you a better chance at a University admission.
Hope this helps
Engraved.