Large Model Aircraft (+drones.) are not allowed above 400ft agl, and GA aircraft are not allowed below 500ft agl.
The not below 500ft agl rule doesn't apply in the UK for aircraft. It is a EU requirement but the UK filed a difference to that regulation at the time it was introduced. The restriction not to fly within 500ft of a person, vehicle, building or structure continued to apply, together with the other elements of the 'low flying regulations. I assume that the drone operator will be applying to the CAA for the most appropriate height/altitude for their operation. It may well be higher than 400ft above agl of course.