I got Regents and Honors at UC Davis but my top choice is UCSB (where I was accepted without any perks). if I email and inform them that I have this offer from Davis but my top choice is UCSB, would I possibly get consideration
No. Each UC makes independent decisions on their Regents and Honors students so UCSB’s criteria is not the same as UCD’s. Congratulations on UCD and Regents.
You can’t appeal Regents but you can apply to be in the honors program at UCSB. I believe you can’t do it until after your freshman year. You might look into the requirements here: https://www.duels.ucsb.edu/honors/eligibility#apply