I think MIssy's killer knew there were cameras inside hence the outfit, and also knew the external CCTV wasn't working hence apparently parking their car there. I also think he/she had been inside the church before which would not be unusual even if it wasn't their church. Someone upthread mentioned the blood drive, pretty sure it was the day before MIssy was killed, Sunday 17th.
I think he/she knew Missy in person, it was a love/revenge thang, and they knew from Missy and BB's FB that he was away and she'd be there alone in the dark. I think the person has connections with LE, the fire service, the Cowboy Church, the gym, CampG or possibly even all of the above. Just "Todays Theory by Cags".