I have read that the wheel brakes on an aeroplane are by for the most efficient means of stopping it. I was wondering whether in low grip situations, such as in rain or snow, an ABS (Anti-lock Braking System) system is used to ensure the wheels do not lock. If not, why is this not necessary?
Yes, they do. They are called Anti-Skid and they go from Mark I of 1948, with simply an on-off switch triggered by wheel lock, till Mark V (or at least that's the last I've seen), with quite complex control systems and sensors behind.
It is not called ABS but Anti-Skid, but the principle is similar. All large aircraft have it.
The purpose is however slightly different. In aircraft the nose wheel has relatively little weight on it and is usually not braked, so directional control, the main reason for ABS in cars, is possible¹ even without anti-skid. However due to the higher speeds and weights involved, aircraft have much higher risk of skidding and hydro-planning and when either happens, it can severely damage the tires, so the anti-skid is to prevent that.
¹ To an extent. Since the nose wheel has little weight on it, it also has limited authority, so if the braking force becomes too asymmetric, the nose wheel can't compensate for it.
The ABS systems in automobiles were based on the systems that had been in use in aircraft for decades.
ABS was first developed for aircraft use in 1929 by the French automobile and aircraft pioneer Gabriel Voisin, as threshold braking on airplanes.
The first ABS system used in a car was the Dunlop Maxaret system, which was developed for aircraft (and used in a lot of British aircraft in the 1950s and 1960s). It was used in the Jensen FF, introduced in 1966. From 1971, car manufacturers introduced purpose-built automotive ABS.