The drone code in the UK is quite specific on the maximum height for drone hobby flying and commercial drone operations. But the devil is in the detail. The Civil Aviation Authority permission for commercial operations states that aircraft shall not be flown ‘above 400 feet above ground level.’
There is good reason for this. The airspace above 500 feet is used by general aviation. But drone users need to be vigilant because military flying takes place down to 250 feet across most of the UK and down to 100 feet in some areas.
Online forum discussions between those starting drone flying have asked ‘if the pilot is stood on a 400 foot cliff and flies the drone off the cliff what height is legal?’  It is still 400 feet above ground level, o the drone would need to be flown level to, or below its take off point (assuming that it is on top of the cliff). Hopefully this explanation is clear in the following diagram.

UK Drone Law – Maximum Height