Don't "believe" - check!

Ensure your registered purposes cover your intended use of the footage and its retention / deletion.

Then there is the flip side - whilst you may record what they are doing can CCTV advise them how to behave, what they should be doing, intervening at the time they start doing things wrong (TOO LATE after the event). Monitoring third parties is more than just recording their activity.

Every time I see an article announcing the arrival of CCTV to "tackle crime" - WRONG, it merely records. You need boots on the ground to influence behaviours, catch perpetrators and intervene before the crime escalates. Same with road side cameras - all they do is identify a number plate and from that information the last known registered keeper who was honest enough to provide details to the DVLA. As we see more low emission zones cloning is becoming a serious issue that cameras alone cannot solve. Cameras do not catch drunks, drug drivers, those without a licence / MOT / Insurance - that needs "monitoring" by the police officers.

https://ico.org.uk/for-organisations/uk-gdpr-guidance-and-resources/cctv-and-video-surveillance/

as the CCTV could also capture your own employees the link above reads to some suggested reading