“Have a guess at my mark (score) in English.” “70?” “No, A HUGE/BIG MISS.” Does this sound natural to mean it’s not even close or you’re way off?

Why not just say "not even close" or "you're way off"?

I understand what you mean by “huge miss” or “big miss”, but it’s not something I’d personally say. That doesn’t mean it’s wrong, though

It’s sounds more natural if you’d say, “No, you’re way off.” At least to this American.

If you said either of those I'd think you got a 0. If you simply failed, something like "I wish" would be more fitting.

I would say “way off.” Or just “ no/wrong” I would not say miss unless I’m hosting a game show

[US] I would simplify the first sentence to "Guess my English score."

If the score is out of 100, I would not consider any guess to be way off. To me huge would be like hundreds or thousands of. But I wouldn't say big miss either way. I'd say: not even close.