Hello, I think goldtoof gives you good advice upthread, I'd take it on board.
Take the test, get it over with and forget about it.
Yes it is ridiculous you have to do it, but just smile and nod and don't give them reason to think you are non-compliant.
I have a slightly different experience in that I have been open about the level of anxiety I have in this PG (have lost the previous 5 pg's so it is hardly surprising) and like you I really wish I'd kept my mouth shut now.
I think the problem is that the MW meets you for so short a time, and makes a judgement just on that short time and things that have been said in the meeting, not your life as a whole which they don't get to see.
I also think there is some ridiculous ticking of boxes going on- mention of anxiety- quick, refer. Mention of drugs- quick, refer.
My friend even got a social services referral for booking in late in this pregnancy as "pregnancy concealment" is a big red flag. (They seemed completely unable to grasp that because she lost her last PG at 15 weeks and was then chased up by Midwives who hadn't updated their notes regarding her loss, she naturally didn't want to start the ball rolling until she thought the PG would be viable).
The thing is, when you see them, they give you a lot of talk about how they want to support you, help you etc, and that may be true, but they are also plugged into this larger system and it has all sorts of other stuff going on in the background.
As you have said something that has obviously ticked one of their boxes, I think you have to humour them, because not doing so would be seen as more evidence that there is a problem.