This is because the UK Border Force's role primarily focuses on controlling entry into the UK, ensuring that individuals entering the country meet immigration requirements and comply with customs regulations.
Instead, departure procedures at UK airports are managed by airport security personnel and airline staff, which ensures compliance with aviation security regulations and facilitates a smooth departure process.
However, travellers departing from the UK may still go through security checks conducted by airport security to screen passengers and luggage for prohibited items and potential security threats. Additionally, travellers may also go through customs checks if they are carrying goods that need to be declared or if there are suspicions of illegal activity.