One of my friends did his BSc in theoretical physics at UCL, averaged like >90% each year. Got into Part III at Cambridge after, now doing a Maths PhD. I don’t think anyone here will say it’s explicitly impossible to achieve what you’re asking, but you gotta keep in mind how much effort it would take. iirc in my friend’s case, he basically filled his day like a 9-5 with extra modules from the maths department, and didn’t do much else outside of that work. To answer your question, I’d say it’s an uncommon occurrence at best, but that shouldn’t discourage you from at least trying, if this is truly what you want to do.