yes, go ahead to take it now or in future. some govt jobs asking for good English grades for O level.
A-maths is a must, not necessary to re-take english
off-topic what is ur gpa and what course are u studying
C6 ok liao lah
so old liao then wanna pia language is quite difficult
even if u retake oso max go up 2 grades nia
Getting C6 for O Level English is not ideal but should still be fine. I know a lot of people who got C6 in O Level English and who still got into NUS/NTU. A Math is a valid concern for you, and I guess you should try re-taking it if you are seriously interested in getting into the NUS Computing course. But if you decide to take A Math again, do be prepared to spend time each day practising some questions from the textbook. Hire a tutor or join a tuition centre if you need to.