Do children still have to sing Jesus anthems in Uk schools?

We didn't at my school. I think that was decades ago in the UK. Unless you go to a religious school.

Not these days in state run schools.  Christianity has been watered down in schools in the U.K. so that it seems virtually meaningless to the children. They are taught about many religions and are not allowed discussions.  My granddaughter was forced into saying something that she had not been taught at home and bullied by the teacher until she answered YES.  The question was "Are Jesus and Mohammed the same?" She answered NO and the teacher repeated the question over and over and over again until my granddaughter said YES and then the subject was dropped.  If a muslim child had been forced into saying something that they did not believe there would have been an outcry and parents screaming at the school gates and going to the press about it.

I was the one who called the police. They told me I was a bloody good citizen. In the UK these days it is more likely that kids sing muslim songs. 

in private Christians schools...

In catholic Primary schools yes. Not sure about other schools.