Here's the CPS guidance on bad character evidence.
www.cps.gov.uk/legal-guidance/bad-character-evidence
Carl Beech's convictions would be admissable under gateway 7, because he made attacks on the character of others. However, I'm not sure if this only applies to others
involved in the trial, ie one of the men he accused giving evidence as a prosecution witness for example.
The issue regresses to the basic question of how do we know Mr Beech was making it all up? An accusation on its own, without any supporting evidence, has been enough to convict tens of thousands of men (including our host, to name but one). How does the CPS distinguish between Carl Beech and any other historical complainant who is treated as a genuine victim?