I'm a shopkeeper. If someone walks into my shop, picks an article up and walks off with it, with no attempt at concealment, the law does not allow me to physically apprehend that person in any way, even a tap on the shoulder. Shopkeepers have been convicted of assault when chasing shoplifters, sometimes concurrent with the thief being convicted of theft, but sometimes it's only been the shopkeeper or shop staff being charged. This scenario has never quite happened so blatantly with me in attendance, but it has with female staff and I would never have expected them to pursue.
Several years ago a Woolworth's security guard (yeah, that long ago!) was pursuing an alleged thief along my road, calling for someone to dial 999, which I duly did, while all the time being attacked by this individual. He did not retaliate, other than trying to defend himself. I'm happy to say the police responded quickly and arrested the shoplifter.