This isn't an easy one to answer - whilst work experience is definitely seen as a plus by employers, doing so without a background in the subject means you *may* get less out of it than someone in say their second or third year at university will.
Historically, Mathematical subjects, e.g. (Engineering and physics) don't regard a gap year as necessarily a good thing - your mathematical skills do degrade fairly rapidly if they're not used, and so people who have had a year out from A-Levels (or equivalent) may find it harder to keep up with the rapid rate at which you learn new maths in your 1st and 2nd years on most Engineering degrees.
That said, if you can find a company that will challenge you technically, then this will give you a very good head start into the degree - certainly in writing reports and understanding how concepts relate to the real world. However, in my company, we don't even take 1st years for summer internships except under very rare circumstances, due to the level they're at in general. I did a summer internship after my first year, and it was a far steeper learning curve than internships I did in subsequent years of my 4 year degree, and I didn't really get as much out of it due to my lack of general knowledge around the subject - of course you learn, and learn quickly, but the level of support needed may be more than an employer is willing to offer.
So it's down to you. I had classmates who had done industrial gap years prior to joining, and they reccommended it - they had some skills (and some money they'd earned - always useful as a student) already. However, post degree they were at the same level as the rest of us but a year older, and only marginally more employable than those of us who had spent a summer at a company.
The big plus is that it helps you realise whether or not the sector is one you wish to go into. An engineering degree is hard work, and if you've got some experience in it and know that it's what you fancy doing in life then it makes it miles easier to determine what to study at university.
Alternativley, do check out apprenticeships in engineering - they're a real alternative to a degree, and once completed you'll have far less debt to contend with (but you won't get the university life experience)
Kind Regards,
Stu Haynes, MEng