I wear a partially Hi-Viz textile jacket for commuting into Manchester daily. It also has reflective strips. I don't know if it does any good, but it can't do any harm, and as JP said, if anything goes wrong, at least you have a stronger case.
And with a big BMW tourer, and a Hi-Viz jacket on top, you would be surprised how many car drivers pull over on the motorway. I suspect even more would if I had a white helmet
John