UK, can my employer refuse to pay us because we didn't "clock in"

As title really. Employer is a large UK company. Management are completely incompetent and have now posted a notice stating that if we do not clock in we will not be paid. I am of the understanding that this is illegal in the UK, can anyone clarify? They refused to pay me for contracted shifts I have worked before until I proved I was there with log ins we have to do in order to complete our job.