Some things to be aware of (and consider); in the UK you will also be "graded on a curve" to a point. Usually in the summer after exams, the universities' exam board will meet and discuss moderation and they will consider how the grade distribution is compared to what would be expected and use this along with looking at the exams to decide if things are too hard or too easy in a particular exam, and moderate the grades of students up or down as applicable. Also just doing an MEng does
not by itself qualify you for CEng status. The MEng satisfies the educational requirements for CEng status only; you then need to complete the workplace learning and experience and build up a professional portfolio to fully qualify for CEng status.
In terms of the "campus" thing that's a bit irrelevant as applied to Imperial because Imperial isn't a campus uni, it's a city uni. The uni is embedded in the city of London and so the entire city is the "campus" to a point. Thus if you find London boring then I don't think you'll find anywhere in the world interesting, and that might be more to do with your perspective, than the location.
@Sinnoh might be able to comment on social life things at Imperial (albeit not in the civil engineering course/department specifically). My general experience of social life at uni is that it is always what you make it. Students can and do have very active social lives, even while doing well in demanding programmes, at any uni. Whether you make that work or not is up to you, not the uni.
I don't really understand the relevance of the Silicon valley proximity if you're studying civil engineering. I imagine London has plenty of (possibly more) civil engineering oriented roles, compared to NorCal, as well. I'm also skeptical about your statistic on summer courses, do you actually have a source for that? Also while you will have some "time off", once you leave school you really can't get away with spending 1+ months not doing any academic or actual work, if you want to graduate with good results, on time, and with a good amount of work experience to make yourself employable. Over the summer you probably should be aiming to be in internships/work placements to make yourself more employable on graduation, while in the winter and spring breaks you will probably be spending at least some of that studying in preparation for exams (certainly in the UK at least).