I really don't see why you can't study physics at Cambridge and yet still become an inventor/writer in your spare time.
Look, being a writer is a problem. Almost all writers make a pittance of a profit. Most have to have a proper job as well to maintain an average lifestyle(usually involving the academic world for obvious reasons re spare time and holidays). I would suggest signing onto a small publisher as they are doing great, from what I hear, whereas the larger ones are suffering a bit.
Being an inventor is also difficult. If you try to get money to fund your invention, then ,a lot of the time, the business concerned will, in the end, try to grab your invention as well. Getting a patent is also a nightmare with people in some other countries stealing your idea without paying because they know you won't win in the courts - I know of some examples of such.
Overall being a physics lecturer/professor would be a great job. Unlike the soft sciences, hard sciences like physics haven't been as poisoned by Marxists like Derrida yet, so there's more academic freedom. Plus, you'd have all that free time outside classes and school-holidays(minus time needed for marking essays/papers) to pursue your dreams of becoming a novelist/inventor, and having a little extra money from other pursuits would be nice. If you eventually succeeded wildly in the latter options, you could then give up the day job and devote yourself exclusively to writing or making inventions.
Not having a degree means a lot less than it used to. However, some countries like Germany and Austria, for example, still revere degree-owners and you're basically socially dead without one.
