Hi Scitech
In the UK at least, it is unusual for a science student to propose their own thesis topic, rather than apply for an already funded topic.
However, I would suggest looking at departments whose work interests you, read some papers by the staff there, and see if that inspires you. It is easier to develop hypotheses/research questions on the basis of well defined gaps in the knowledge (as are sometimes detailed in the discussions sections of papers) than it is to come up with something from scratch.