Sounds like sensory overload to me. DD1 has SN, and part of it is that she can get overwhelmed by everything going on, which leads to meltdown. Looking back, it is hard to identify one sole trigger, but we then realise it is a 'straw that broke the camel's back' situation. Does that ring bells?
The other things you mention, "She is a lovely, sweet, able girl otherwise - when she wants to be. No other worries other than a lack of regard for authority figures."
do sound a bit like aspergers/ASD. It isn't a lack of regard for authority, but more that a child is on their own agenda, and gets frustrated if someone interferes with that.
Perhaps you should ask your GP for a referral to a developmental paediatrician. If she is a typical child, they will tell you. But it would be a shame for her to be labelled naughty if she does have ASD, and there will also be strategies that work to help you minimise the meltdowns
Have you considered minor strategies such as:
- Giving warning with a count down when you will expect change. i.e. "DD, in 5 minutes you need to brush your teeth." "DD, teeth brushing in 2 minutes" "DD, time to brush your teeth now".
-Make sure that you make plans in advance and stick to them as far as possible, preventing sudden surprises.
-Using now & next as key words for activities, so that she is clear that things will happen, just not yet?