I'm speaking specifically about continuing on to engineering at university - in this case Physics is required for almost all courses. Taking Engineering as a third/fourth A-level in addition to Maths and Physics would be acceptable for continuing to study engineering at uni, although Further Maths with as many mechanics options as possible would be the next best choice (or for chemical//biomedical/materials engineering, Chemistry).
Additionally most aspects of A-level Physics will be relevant for parts of most engineering courses; for example the quantum phenomena is relevant for semiconductor physics, underpinning all kinds of computing and electronic engineering topics, the wave topics are relevant both for electromagnetic wave propagation in electrical/electronic engineering and also aspects of mechanical or civil engineering (acoustics, vibrations etc). The materials elements will crop up anywhere in various guises. There aren't really any "non required" elements other than that in theory they could just teach you the whole thing from scratch in a mathematically rigorous fashion, ala the Russian (and to an extent, French) style - this has it's downsides and more relevantly isn't how engineering is taught in the UK
In general, if you intend to continue to higher education, it's most useful during A-level selection to consider first if you intend to pursue a STEM subject or not. STEM subjects usually are best prepared for by, and usually prefer, to have a full set of science/maths A-levels. If you decide you do not wish to pursue STEM areas, but are not entirely sure otherwise, it's fairly reasonable and not too difficult to build up a broader portfolio of A-level subjects to prepare for multiple possibilities. For example, History and Maths would provide excellent preparation to any number of social science or humanities courses, including more quantitative ones such as economics.
While mixing "arts" and science subjects is suitable for continuing to "arts" subjects (i.e. social sciences and humanities of various "flavours" including some slightly more quantitative such as economics, or more scientific such as human sciences or some psychology courses), it's usually less helpful for enabling you to continue to the sciences; certainly at e.g. Cambridge or Imperial, you will limit the courses you could apply to significantly and potentially weaken your application for those courses.
This is an unfortunate truth but it's best to be aware of before you move into A-levels, than after. That said, if you do take a mixed set, or even purely "arts" subjects, there are many science/engineering foundation years available which will allow you to follow this route. Some examples include Manchester, Lougborough, and Southampton.
It's also worth noting 4 A-levels is unusual if one of them isn't Further Maths in a purely STEM A-level portfolio, and isn't actually required for any university. This is also a larger work commitment than it would usually be and should be carefully considered before continuing with; while taking e.g. 3 science subjects and say, History would prepare you for all manner of courses, it's a signifcant workload. Additionally many quantitative social science subjects or more analytical essay based subjects such as Law will accept science A-levels as suitable preparation. not doing subjects such as History or English literature usually only limits you from pursuing those specific subjects, and most humanities or social science subjects otherwise aren't assumed to have been previously encountered and usually have no specific requirements beyond a rigorous academic preparation (which a science subject portfolio provides).
I'm aware the above may seem biased in favour of STEM subjects, but that wasn't the intention; this is just my experience as my background has been in these areas. I had the misfortune of doing IB so I have some background in "arts" areas however so I've hopefully provided at least some insight into that side as well
the main thing is, think about what you may want to do after school, generally. Pretty much, consider if you intend to continue to uni (which isn't by any means required and there are an increasing number of degree apprenticeships that allow you to earn a degree in many areas, including law or engineering for example) and then consider if you want to continue in STEM, or otherwise. This will be a good first step to take