Hi, I'm a UP graduate as well. I didn't attempt to ask for a Certificate of English as Medium of Instruction because a lot of my classes were done in Filipino, really.
You might want to read this as well:
<i>
Medium of Instruction
English is generally used as the medium of instruction in the University.
The UP Language Policy provides for the development and use of the Filipino language while maintaining English as a global lingua franca. <u>The Policy states that Filipino shall be the medium of instruction in the University at the undergraduate level</u>, within a reasonable time frame or transition period. Graduate courses of study shall be in English, though there could very well be graduate courses of study in which the medium of instruction is Filipino.</i>
Source: http://upd.edu.ph/admissions/academic-information/
So yes, you'll have to take the IELTS. Good luck!