Good choice (MORSE)!
To be honest, the department is unlikely to be that bothered about your EPQ.
I did one on the calculus of variations, which was quite useful for me personally, but I wouldn't say it improved my chances of getting in for MORSE much at all.
To get a place, my best advice to you is to just make sure you get A* grades in maths, further maths and one other (excluding general studies and critical thinking).
You may also want to look at the incentives that are in place for candidates who do very well in STEP/AEA, as these can be quite generous!
One other thing: economics at university is probably fairly similar in year 1 (apart from there is more maths), but from year 2 onwards it becomes much more abstract as MORSE people focus on mathematical economics rather than the cliché micro/macro combination. Though weirdly enough there is not much focus on econometrics in MORSE, unless you want to specialise in that later on (which you can do).