OK. A bit of misiformation. Business studies as far as I know isn't related to engineering. The only bit that I could imagine being relevant is the accounting in business.
Business studies as a subject isn't a required subject. So if you're saying that business studies would widen your options for picking a degree, you're incorrect there unless you can provide evidence to say it's a required subject.
Business studies also isn't a required or a requested subject for any career or role that I could think of. If you plan to go into a business role, you can usually do so with a degree in any subject or without any prior qualifications. Those business related qualifications that do count would be the professional qualifications (e.g. accounting, HR, marketing), and most of them don't require any prior qualifications to get into them or just A Levels in any subject (irrespective of whether you have business studies or not).
The thing I do like about business studies at A Level is that it gives you a good background in business should you wish to start your own business. You also repeat a lot of the same material that you covered at A Level in a business degree (bachelor's, master's, HND, HNC/foundation degree, etc.) - PhDs are different.
Having said that you can go into business or any business role with an engineering degree. In my opinion, you're not really gaining much by doing business studies unless you intend to go into business yourself.
If you want to broaden your options, I would pick an A Level subject that is a required subject should you wish to broaden your choices of degrees to apply for e.g. chemistry, biology, History (sort of), Geography (sort of), English Lit (sort of), psychology (sort of), Music, A language (modern or ancient), Classical civilisation.
Biology + maths would allow you to do a degree in bioengineering. Chemistry + maths would allow you to do a degree in chemical engineering. 2 sciences (or 1 science+1 maths) tend to open you to a number of healthcare and science related degrees.
If you want to maximise the number of careers that you can go into (not always a good idea, but hey), then I usually recommend the 3 sciences + maths. The sort of careers where you're expected to have a degree level education prior to going in that requires specific degrees tend to be those in healthcare, academics, and teaching. For the areas where you can go into with a degree in any subject includes law (solicitor). For areas where you need specific types of degrees but you can go into via apprenticeship includes: medicine, architecture, engineering, economics, actuarial science.
If you want to get the specific entry requirements for specific careers, I recommend looking into:
National Careers Service: https://nationalcareers.service.gov.uk/explore-careers
Career Pilot: https://www.careerpilot.org.uk/job-sectors/sectors
There are about 800 different careers to go through, so you would be kept busy if you went through all of them.
As I have absolutely no clue what else you want to do outside of civil engineering, I would need more information in order to clarify any of the above if you want to be specific.