When it comes to engineering, the actual university is not the most important factor that employees look at - Many on TSR would condemn those who could to low ranked universities, but in engineering they have an extremely good chance of getting a job like anyone else. So don't think of not going to Cambridge as damaging your prospects, but it certainly would be good if you could get in.
Regarding the year in industry... It is extremely valuable, but it is not a guaranteed venture. Summer work will tend to be limited as you only have 8-12 weeks and so it is difficult to get hugely involved in a project. A full year will allow you to progress much further and perhaps manage your own work instead of constantly being stuck in the "learning" phase.
However, placements are not easy to obtain. You may be going on a degree with a placement year, but it is going to be down to you to secure work (the only degrees I am aware of which guarantee it are when they are a part of the course, such as NHS sponsored courses). I am doing mechanical engineering and I started applying for placements in September and it took until May to actual secure a job. I have applied to close to two dozen companies and had interviews with a handful of them (a couple of assessments as well). There is every chance that you will not get a placement, so don't assume that you will.
That said, if you can get work of any sort, it will often lead into employment. This is very much true with full year out placements, but if you secure summer work routinely with a company, you may be able to get graduate work with them.
To me, I am not personally bothered about where I studied so long as the facilities were good - I never did it because of the name on the entrance sign. You may have more preference to get into Cambridge, and that is understandable. Getting work is valuable, but it can take a lot of effort and still land in failure. I still believe the year in industry will be worth if it you can get a job though.