A collective agreement which required jobs in a particular part of a factory to be given only to men would be void, so a woman who applied could not be refused on those grounds.
An indirectly discriminatory rule of a qualifications body (providing for example a professional qualification for plumbers) which required that applicants must have two years’ previous experience with a British firm would be unenforceable against a person who had the equivalent experience with a foreign firm. It would still be enforceable against a person who did not have the required experience at all (provided it was justified).