Are you still planning on applying?
If so, I would suggest taking some Philosophy courses or otherwise reaching out to philosophy teachers. One of the requirements is to provide two samples of work (2.5 to 5K words each). So you will have to practice long-form philosophical writing. It would be great if (either through being in a course or having a friendly philosophy professor) you had those essays checked and assessed by somebody in the field. It would help you improve the essays so that they are up to the standard.
It would be good to pick up some introductory books about the ethics of AI and read them, to get to know more about the subject.
Also, another requirement is to submit a research proposal, so you will have to have a good enough idea about the ethics of AI in order to come up with an idea of something that hasn't yet been researched that you would like to research yourself during your MPhil.
One way of going about it, is to firstly read an introductory book into the field, see which topic interests you (there are many topics from consciousness, intelligence, governance, AI bias, etc.) and then delve deeper, while trying to note down the questions that come up in your head. Then you could discuss with your philosophy professor if those questions have been answered and if they could serve as the basis for further research.
You could check the CFI Cambridge's MPhil modules currently offered for an example of the different categories that they use.
They look for people from diverse fields, so they will be happy to see somebody with a computer science background applying.
Cheers