Take a gap year for engineering at Cambridge

Doubtful. It might be valuable for you yourself of course (although as below I think actually in the engineering sector, you're much less likely to find anything substantive), but ultimately the overriding consideration for Cambridge are going to be your academics including as demonstrated in interview.

Also "year in industry" usually refers to a placement year during a degree. It's really unlikely you would do much in the engineering sector as a school leaver on a gap year beyond shadowing or admin type activities. They aren't going to let an unqualified 18 year old touch any of the engineering design or modelling aspects whatsoever I would anticipate.

If you want to take a gap year for your own sake though by all means do it - don't do it because you think you'll get "bonus points" when applying to universities somehow, as in almost no situation is that going to be likely.

Thanks!