Personally, I am convinced that Andy will not get in any professional bother. He is doing exactly what he is employed to do. The bank wants to demonstrate its credentials on social media to the kind of people who look, and he's doing that.
Less than 20% of the UK use twitter. Of those that use it, I think they are more likely to subscribe (however weakly) to the concept of gender than to find it problematic. People who find it problematic get banned...
The only people who are going to make banking decisions on the basis of the tweets are committed feminists and trans activists, because changing banks is a load of hassle.
However, on either 'side', for the want of a better term, there are only about 5000 twitter accounts who argue about it all on a weekly to daily basis.
Of the 5000 GC accounts, it will only be a fraction that banked with Halifax to start with, and all they will do is move banks. Halifax can afford to lose those accounts. But if you piss the TRAs off, they will do more than silently close any existing account! It's safer to irk female account-holders, especially ones with secure employment and stable finances. They're too reasonable to dress up in black bloc and stand around threatening people leaving Halifax HQ.