Slot prioritization would appear to be the driving force here. ABZ and NCL are also seeing a reduced frequency. BA at MAN is predominately LHR transfer traffic and BA have figured that they can get away by reducing frequency but maintaining just slightly less uplift (Higher seating configs). The argument that MAN doesn't need BA because of our own growing Intercontinental network does not really hold up as has been commented, the likes of KL/AF/LH en all have maintained frequency.
The schedule for winter 19 is showing 8/9 daily at the moment. Expect that to be reduced too. As for Sun-Air, with RYR starting BLL, they'll probably not be around for much longer unless they can maintain the corporate contracts.