The problem is, this employee approached the 14yo offering help. The employee did the approaching and that is the issue here. If your girls asked a transwoman for help, that would be different.
I do partially agree with you that on this website, there is a lot of anti trans stuff, dressed up as feminism. When it is in fact pretty hateful. The problem is that ordinary trans people ought to be left alone to live their lives. But there are a minority of trans people who act in way that is unacceptable. And they cause problems for everyone by doing that.
In this case, what happened was absolutely not on. This employee should not have offered a female child help with bras.
I went into M&S lingerie with my teen dd a few weeks ago. There was a trans woman (customer not employee) wearing a short pink dress - height around 6ft3 or 6ft 4. I have to say, I was uncomfortable. I don't care about men shopping with women when women are buying a bra or whatever. But this particular individual didn't actually need a bra (no boobs), and didn't buy anything either. Was just wandering in the bra section. I didn't buy anything and I felt quite uncomfortable.
Bra shopping, especially for a young teen, can be very embarrassing. M&S of course should employ trans people without discrimination - but it's pretty obvious that the bra section isn't the place for a trans woman employee. It's a bloody department store - so many places to work other than bras.