Hi Tracey
It's understandable your concern about having one very large remaining breast. Physically it could be very challenging for your body to adjust to the asymmetry, posture-wise etc. Psychologically it's a pretty personal thing. A woman fitting me for my postsurgical bra said she'd had cancer and a mastectomy but really wanted to keep her remaining breast. I think your team, especially if it includes not only the surgeon but your psychologist and breast care nurse, will have your best interests at heart and advocate for you. You do have a right to say what you want, but they have an obligation to you as well.
I had a bilateral mastectomy about 9 weeks ago - my boobs were 12DD and am really appreciating my new smaller speed bumps. Much more aerodynamic and no more mammograms.
All the very best :)