According to this "Guardian" article by their media editor Jim Waterson...

BBC News staff told not to tweet personal views after LGBT debate

"On Thursday the director of news, Fran Unsworth, told staff to stop expressing strong political views on social media and to stop publicly criticising colleagues, warning that they could face “appropriate action” if they breached the rules in future.....

Among other issues, Unsworth told staff they must not state their political preferences or say anything that compromises their impartiality or criticise colleagues."