That is pretty awful, and although it's legal to argue over contract terms, including the employer being completely obstinate and your only option being not to agree, you were right not to back down and accept such a one-sided negotiation.
In the case of SPMs, as the Post Office holds all the cards, they can kick out anyone not willing to accept the unfair contract... like the Mr Bates at the centre of it all, who lost his position as SPM after standing his ground and refusing to accept liability for blatantly false losses attributed to his branch by the Horizon software. Not every SPM was lucky enough to be in the same position as him.
In the case of SPMs, as the Post Office holds all the cards, they can kick out anyone not willing to accept the unfair contract... like the Mr Bates at the centre of it all, who lost his position as SPM after standing his ground and refusing to accept liability for blatantly false losses attributed to his branch by the Horizon software. Not every SPM was lucky enough to be in the same position as him.