I'm very charitable so perhaps that's why we disagree here, but I didn't read his comment as racist personally. That said, I understand the use of the word "monkey" could have been used with racist connotations, but the burden of proof would be on you to proof that instead of assuming guilt.
But either way he wasn't charged with being a racist. And outside of being a member of an extremist group I don't believe it's actually illegal to be a racist in the UK. It's illegal to be offensive. And that's what he was ultimately prosecuted for.