Hi there! This sounds like a really sticky situation to be in. My personal advice would be to have a think about why you regret not applying to a 'top uni'. If it's purely a case of worrying about future prospects after your degree, many non-russel group university graduates have gone on to do amazing things too. For example, the current prime minister of Serbia went to the University of Hull! Many graduate jobs/schemes care more about what you do with your degree rather than where you go so maybe looking at things like student internships, summer jobs or volunteering projects could be a really good idea to broaden your horizons a bit. I'd maybe also recommend maybe talking to your student adviser/the careers team at your uni as they will have really good resources and information about opportunities whilst you're a student.
This being said, if you genuinely hate your degree and want to go somewhere else, then it's not too late to transfer. I'd recommend (again) talking to your student adviser as they can support you better in this journey than I can - as well as doing some research online about other universities to see where you'd like to go. Maybe get in contact with their admissions teams too to see whether they'd accept your application/ how you should apply etc. Some unis want you to go again through UCAS, some may just want other documents etc sent via email so contacting them could be a good start.
Either way, good luck in however you choose to live your future.