The overheads are indeed smaller than A320 series aircraft, but can get a bag that conforms to BA's measurements in them. There are overheads over the wing area in the middle of the aircraft which are half height or so, so ok for laptop bags or briefcases, but not rollaboards. There is a fair amount of equipment stored in the front of the aircraft, so the overheads for rows 1 to 3 are a bit constrained. Usually there is gate check in of bags for LCY unless, say, the aircraft is half full, in which case there isn't usually a problem.