Better to be a teacher in a relevant subject. Counselors rarely have the ability to give examples of how your work impressed them. Note it is very different to the usual US reference style (give them the UCAS/Oxford guide for references), and ideally needs to say (for Oxford) some paraphrase of “X was the best student I’ve taught in this subject in the last 5 years”.
It’s not quite the same but academic references (ie your undergraduate professor) for Cambridge postgraduate courses are asked if a candidate is “exceptional, strong, not strong, weak”.
The definitions are:
Exceptional: best student in last 5 years
Strong: best student in the year
Not strong: top 5% in the year
Weak: not in top 5% in the year.