Congratulations on your pregnancy, and for maintaining an excellent HbA1c!
Re the dietician, YANBU. She doesn't seem to know what she's talking about. You can certainly refuse to see her again.
On the other hand, there might be legitimate reasons why a small increase in carb consumption is necessary in pregnancy, and that good control is just one part of the equation.
I'm not an expert on this kind of thing, but I was told by a doctor once that a fetus needs carb for brain development - however, it was many years ago and may no longer be the current thinking.
Nevertheless, a low carb diet is not the norm in pregnancy (since most pregnant women aren't diabetic). So in your shoes I'd want to know if there might be any adverse effects of a low carb diet, despite the positive effect of good control.
If I were you I'd ask the GP about getting access to research / information about nutrition and fetal development - maybe a referral to a consultant who specialises in diabetes and pregnancy?
Really sorry if this isn't helpful, and feel free to ignore me completely if this isn't the sort of thing you want to hear about.