Under the new regulations that came into effect on the 30th July 2018, all unmanned aircraft are restricted to 400ft from the surface (above ground level) without specific permission from the CAA or anywhere within the flight restriction zones of a licensed aerodrome without permission from air traffic control.
Shown below are the flight restriction zones for London Oxford Airport. Any operation of an unmanned aircraft within the red shaded areas is prohibited without the permission of the air traffic control.