People are being brutally honest.
If you’ve seriously had a dream of Oxbridge for years, you’d have worked for it both undergrad entry and during your degree. But you didn’t. Paying out of pocket for a second degree for one year of Oxbridge study, which in itself is unlikely, and at this stage not really necessary.
If you want to work for a second degree, get a first (which is something that will require a heck of a lot of work) to then make an Oxbridge application, do it. If you want to, go ahead. It’s your future, and it’s your call.