When i lived in the UK (same laws apply) I was driving behind a Jag XJS which was all over the road and I mean ALL OVER THE ROAD and was prooving a real hazzrd to himself and everyone else.
I used my mobile to ring the police while tailing him.
I have the details of his car and told them exactly where he was, and were we were heading.
I gave full details of myself (as required) and made no attempt to hide the fact that I was using a phone while driving.
They did not arrest me. They did not caution me in any way.
Make of that what you will.