got highly commended in British biology olympiad, can I can put on my ps?

You can put your shoe size in your PS if you want. What matters isn’t your position/awards (those belong in your reference) but how you reflect on what you enjoyed, what you found interesting, what you found difficult and how you found solutions to that difficulty, how you tackled problems etc etc.

Your PS is about you, your academic interests and how you learn. It’s not a list of plaudits.