For all top universities for computer science, further maths is highly desirable. Equally for engineering, it won't be seen as just an addition to maths. Eg you're unlikely to get into Cambridge, Imperial or Oxford without it (unless your school doesn't offer it) for both Engineering and Compsci. At Bristol, York and St Andrews, they even drop the entry requirements if you have further maths so take it. Maths is essential and for most one science subject is also necessary - for oxbridge - this has to be one of Chemistry, Biology or Physics, but at Imperial it can include languages economics computing and electronics.
If you can cope with it, take Economics as a fourth a level. If you're on par with another candidate at interview, you want something to sell yourself with. An extra a-level isn't vital but if two identical candidates are at an interview, one with 3 a-levels and one with 4 - the one with 4 is more likely to get chosen. If you do choose to drop it, just say how much you love maths and wanted to focus on maths and that should stand you in good stead.