Hi, I had similar dilemma as you did. If your good in essays then you should change computer science to economics. Although economics is really content heavy as well as writing heavy. However if you are more on the numerical skills side, you can do further maths instead. I'm currently doing maths, further maths and physics but used to do economics instead of FM and also the same path as you are (mech eng). Also, do some research on the unis you're planning to apply and check any requirements they would want for your 3rd subject