TMUA is designed to be easier than MAT and STEP in terms of question difficulty. It's also taken before A-levels unlike STEP/AEA, so you might have your offers reduced/admissions test part of the offer fulfilled going into A-level exams, which you might feel takes off some pressure. TMUA is administered by Cambridge Assessment, so the centre that registered you for STEP should also be able to register you for TMUA.
AEA's usefulness has dwindled a bit (STEP/MAT/TMUA having become the standard tests) but since it's pretty accessible to people getting high grades in A-level maths and you're doing STEP, I doubt it will distract much from your preparation.