What is mega in British slang?

Excuse me, could you please enlighten me on the meaning of "mega" in British slang? I've noticed its use quite frequently in conversations and online forums, but I'm not quite sure how to interpret it accurately. Is it a synonym for "extremely" or "very"? Or does it carry a more nuanced meaning that I'm missing? I'd appreciate it if you could provide some context and examples to help me grasp the full extent of its usage in British slang.