Police, or any other uniformed officer should always have an easy and obvious means of identification so members of the public can provide feedback about them to any relevant authorities. Any officer who tries to disguise his identity should be removed from duty immediately.
Why does this idea matter?
Police covering or removing numbers and masking their faces while 'kettling' protestors against the G20 summit showed that they weren't willing to take responsibility for their behaviour. A large number of officers doing this gives the impression that their behaviour was planned, or even ordered from above. The Ian Tomlinson incident, and the official misleading statements, made before the cameraphone footage was presented, has given more ammunition to the conspiracy theorists, and helped erode the remaining trust between the public and the police. The Police need to be accountable, and for that to happen, they need to be more open. A policeman hiding his face and/or number should be regarded as out of uniform, and only have the powers any member of the public has. Here in Yorkshire there still remain stories of soldiers in un-numbered police uniforms being deployed during the miners' strike, which shows what distrust it can cause.