Need help with lens.

I recently bought a blackmagic pocket 4k and need help with lenses to buy. I am going to use this camera for university. My budget is around £100

That depends very much on the kind of work you want to do.
But for that budget, you'll have to look for used MFT lenses (micro for thirds).

My budget is around £100

Eeesh!

You should seriously consider adding a zero.

Kamlan makes MFT lenses in your price range.