Mostly BA isn't flying to Red countries, ban or no ban, and if it does then it will be a cargo flight in one or both directions. The Amber service is probably going to be restricted if the Green list expands. From a personal risk point of view I don't think it's significant. We now know COVID-19 is an aerosol and droplet transmitted disease, droplets get to the ground fairly quickly (minutes), and aerosol survival of the virus is fairly short too (under an hour). Though there are a few proven cases occuring a few hours after the reference case left the building, and some from surface contact, this isn't the way the virus generally spreads - it usually needs a new host ideally within 15 minutes, in a poorly ventilated space. The fact that passengers and crew are masked is a further barrier for the droplets and aerosol vectors. Transmission between different flights will be very rare.