There are various weight classes that other countries use which your book may reference, but are not active in the UK:

Fairyweight: 150g with no size limit
US ant: 1 lbs
US Beetle: 3 lbs
Raptor: 6kg
Hobbyweight: 12 lbs(?)
Sportsman: Hobbyweight (?) with a special rule set
Lightweight: 25 kg
Middleweight: 50 kg
Superheavyweight: 150kg