I am no expert (to say the least) in this area but what you are experiencing is why the Fair Cop campaign is so important. I will say what I think the position is and am happy to be corrected because I am as confused as anybody.
of course it's not illegal to refuse to add 't' to the title of a group. I do not know of any law that exists that would make it so.
On the other hand, if someone posted on facebook or wrote (or perhaps even just says) to someone who identifies as a transgender woman to say that they are not welcome at such a meeting because in your opinion they are a man and therefore cannot be a lesbian then this could be deemed as a hate crime.
This is taken from the CPS's site
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In England and Wales the monitored strands of hate crime are:
racially and religiously aggravated;
homophobic, biphobic and transphobic; and
disability hate crime.
These strands are covered by legislation (sections 28-32 of the Crime and Disorder Act 1998 and sections 145 and 146 of the Criminal Justice Act 2003) which allows prosecutors to apply for an uplift in sentence for those convicted of a hate crime.
The police and the CPS have agreed the following definition for identifying and flagging hate crimes:
"Any criminal offence which is perceived by the victim or any other person, to be motivated by hostility or prejudice, based on a person's disability or perceived disability; race or perceived race; or religion or perceived religion; or sexual orientation or perceived sexual orientation or transgender identity or perceived transgender identity."
There is no legal definition of hostility so we use the everyday understanding of the word which includes ill-will, spite, contempt, prejudice, unfriendliness, antagonism, resentment and dislike.
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But to prove a hate crime you have to show that the 'perpetrator' is motivated by hostility due to your 'category' characteristics.
And, for it to be a crime it has to include physical assault, verbal abuse or incitement to hatred.
But not all hate incidents are crimes - verbal abuse has never been a crime in my experience.
The problem is that the legislation shifts the emphasis to the 'victim's perception - if they FEEL abused etc.
It then has to be fought on a case by case basis which leaves everyone confused, suspicious and wary.
This is why Harry the Owl has taken Humberside to judicial review because this drags the police into investigating cases of alleged hurt feelings and enables them to intimidate people where no crime has been committed.
The law is a mess and open to abuse.