Not true at all according to the ICO website.
An employer can film anywhere at any time, if they have a reason to.
How do you think they catch people stealing or abusing patients in care homes if they tell them where the cameras are?
Employers might monitor workers, but if the monitoring involves taking data, images or drug testing they have to do this in a way that's legal and fair
www.gov.uk
`Data protection law covers any monitoring that involves taking data,
images or drug testing. If workers are unhappy about being monitored, they can check their staff handbook or contract to see if the employer is allowed to do this. If they’re not, the worker might be able to resign and claim unfair (‘constructive’) dismissal.`
To film requires an explicit statement in the contract of employment stating the reason for filming, how it will be stored, and processed. If not in the contract of employment it goes in the employee handbook and is referred to in the contract of employment.
Failure to do so violates GDPR in a number of places including Part 3 which covers the processing of personal information. Images (covering video footage) is personal data. As stated, an employment tribunal will not award compensation for the violation (but might award it for other issues such as unfair constructive dismissal). However a solicitor will go for it under the human rights act which does get compensation.
Furthermore, the employee can instead get the correct letter from ICO and issue it to their employer notifying them of a Data Protection breach. Failure to rectify the breach, and give compensation, means they can then make an official complaint to ICO and get it dealt with that way. Regular filming without the right to do so, but for non-nefarious reasons, will get the employee up to a maximum of £7,000 in compensation. For nefarious reasons up to £59,000.
Your understanding on this area of law is inaccurate and worrying because I suspect you have fallen into the trap of doing it. If so, please cover yourself. If you don`t believe me have a chat with your HR department or solicitor.