Aerodrome - Any area of land or water designed, equipped, set apart or
commonly used for affording facilities for the landing and departure
of aircraft.
From the CAP493, MATS part 1, glossary.
Though also from the MATS part1,
NOTE 3 For fixed wing flights taking off or landing at aerodromes within Class B, C or D airspace the reported meteorological visibility at the airfield concerned shall be taken to be the flight visibility.
I am assuming this is said because helicopter met vis for being in vmc is the commensurate with the forward velocity, and no one thought of balloons.
http://www.opsi.gov.uk/si/si2007/20070734.htm#sch1d25
Apears to be the latest Rules of the Air, and it seems to be rule 26 that applies.