You need to warn people that you have put up CCTV in the UK. The UK's General Data Protection Regulation (GDPR) requires that you inform people when they are being monitored by CCTV. You can do this by putting up CCTV signs or notices that say "CCTV in operation" or "You are being monitored by CCTV." You should also make sure that your CCTV system is registered with the Information Commissioner's Office (ICO).

Here are some additional tips for complying with the GDPR when using CCTV:

• Only use CCTV for legitimate purposes, such as deterring crime or protecting property.
• Make sure that your CCTV system is only recording people in public areas or areas where they have a reasonable expectation of privacy.
• Don't keep CCTV footage for longer than necessary.
• Destroy CCTV footage that is no longer needed.
• Give people access to their CCTV footage upon request.
• Cooperate with the ICO if they investigate your use of CCTV.

If you are unsure about whether you need to warn people that you have put up CCTV, or if you have any other questions about the GDPR, you should consult with a legal advisor.