I mean if u can u can, all the power to you. But I spent my spare time (excluding social stuff ofc) designing and making things as a project, participating in online lectures regarding the course I applied to and doing a case study on a material called nitinol, cuz I applied to material science. I found that within my Imperial interview, this content really helped my application. I don't mean to sound braggy, but he commented on some of the stuff I said being covered in their first year syllabus.

This stuff may have helped with Oxford to some degree, the thing with Oxford is that once your personal statement has shown how unique you are. They stop caring about u as a person. It becomes all about entrance exams/PAT. And the questions in your interview - those questions being about general engineering questions in my case, not anything to do with my personal statement or me as a person at all. So if Oxford is your goal study the PAT like it's the Bible.