It depends what you want to do after you leave school. Are history and general studies important subjects for this? If so (or for the satisfaction of getting even better grades), resist year 12. If these subjects are sort of on the side, and the subjects in which you achieved B grades are what you want to do in life, then carry on to year 13. Either way, it appears that you need to find a new sixth form, in which case retaking year 12 is highly recommendable in order to settle in well first.