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The company I applied for said; “Please bring your passport, brp card and national insurance number. It must all be the original copy and not a picture on your phone.”
As for not taking it with you, I suspect they'll just ask you to provide it separately. They'll definitely need it before you start work, and may need it (as I've just said) to do background checks. So if you don't have one (or don't have a record of it), now's the time to sort it out. See Find a lost National Insurance number.