From the UK's Manual of Air Traffic Services.......
1 Vortex Wake Categories
1.1 The UK categories differ from the ICAO vortex wake categories used for flight planning purposes. In the UK, aircraft are divided into four categories according to their maximum total weight at take-off as follows:
Heavy: 136 000 kg or greater
Medium: less than 136 000 kg and more than 40 000 kg
Small: 40 000 kg or less and more than 17 000 kg
Light: 17 000 kg or less
1.2 Helicopters generate more intense vortices from their rotors than fixed wing aircraft of the same weight. Therefore Sikorsky S61N and larger helicopters are included in the Small category.
1.3 It will be noted that several aircraft types have been grouped in vortex wake categories which do not conform to those listed above. For example, the B707, DC8, VC10 and IL62 have been classified as MEDIUM as experience has shown that the characteristics of these types conform more to that group.