The law tries to balance the rights of copyright-holders to have a say in use of their works and to make money from them, balanced against public interest activities such as using copyrighted works in education, culture etc.
In England & Wales points of law are extracted from the decisions of cases that come to court. These points of law become precedents which are binding on other courts at the same level or lower. Law is created bottom up, from a specific case to a general principle. The law changes by new cases arising with different circumstances, and by lawyers persuading opinion on which elements of the case are most compelling. Changes in the law are untested until they come to court.
Key words and phrases from the CDPA 1988 are not defined, deliberately so, including: ‘Original’, ‘Substantial’, ‘Reasonable’, and ‘Fair Dealing’.
It is rarely possible to make blanket statements about copyright compliance that are universally applicable. In each case you need to consider the particular circumstances, highlighting any risks, and try to determine whether the act of copying being proposed is legal.