It would be, and rigorously too.
Because unlike seatbelts in cars, mobile phones or illegal number plates an un-helmeted cyclist would stand out like a sore thumb, to the point where no police officer could be seen to ignore it.
If you doubt what I say, try riding a motorcycle without a helmet.
This is a fair point - It hurts me to accept that... but I do.
However, motorcyclists have registered vehicles and licenses.
I accept that the law (if ever introduced) would be applied by some officers, but I don't think plod would be rigorous.
But... as the law hasn't been passed and in all probability wont be.... we can still all chill.
My feeling (even as a left-leaning liberal who is not in love with the current Government) is that there is some sort of Counter-HSE reaction going on socially, politically and even within parliamentary parties.
I stand by my promise to eat my hat if cycling helmets become mandatory in the UK.