Should I still do work experience/volunteering even after I send my ucas application? Is there any point to it since I won’t be able to add it to my UCAS application?
Then you don’t need work experience or volunteering on your ucas application at all. CS is an academic degree not a vocational one - your PS should be focused on academic interests and activities.
Edit: note however that CS graduates have the highest unemployment rates of ANY subject. So you might want to continue looking for work experience and opportunities to develop your work skills for your own employability - you can’t rely on just a CS degree to find a good job after your degree.