You can try fighting. To give you perspective, a lower grade allows them to assess you in your first year and recommend promotion in 1-2 years. Lower PB quantum and slower increment as you might be near ceiling. Higher grade may mean higher PB quantum and higher increment due to the higher ceiling but you are compared with more seasoned staff who can potentially perform way better than you and have higher visibility. Promotion in 3-5 years if you can maintain BBC grades. Basically big fish small pond vs small fish big pond.