It's not easy to get a MMORSE offer because the course is demanding. I don't want to sound rude, but even if you have A*A*A* at A levels, you may still struggle in your first year.
Reason being - MMORSE covers most of the core Maths modules that Math students at Warwick take, core Statistics modules, as well as one Econs module in the first year.
So you'd need to be at least as good in Maths as those applying for straight MMath, plus have an aptitude for Statistics (quite a sizable amount of Math students already don't want to do Statistics at University after studying Probability B which is a first year module, because it's too abstract), as well as able to do Economics.
Hopefully, your teacher's recommendation and personal statement will show that, because there have been students who believe they are smart by getting top grades at A level, and perform badly, and drop out after the first year, wasting an entire year (while they could get decent grades at other Universities).