Power crazy, definitely.
I think I would go down the route of writing a letter. That way you get to say what you want without being talked over and will receive a response in writing.
Unfortunately, this sort of thing is becoming commonplace as schools realise that uniform sales are a way of upping income. The better schools are far more lenient about enforcing it and more understanding of the students' circumstances, but they're pretty much all adopting this approach because education simply doesn't get enough money. 
There is no correlation between uniform and educational performance or discipline. There are valid reasons for choosing uniforms in schools, but these are not two of them.
The British education system is supposed to be free, and while schools are free to choose their own uniform styles, I believe they have a statutory obligation not to make the uniform so expensive that it encourages selection by making a full uniform very difficult for some parents to manage. £24 for a pair of socks is ridiculous and flies in the face of that.
I'd also point out that failure to ensure adequate supply of the new uniform when rules were to be enforced that stringently shows incompetent management and an inflexibility that is totally at odds with the requirements of a good HT. Either the rules should have been relaxed or greater effort should have been made to make the uniform available.
I'd definitely mention how unprofessional you found her interpersonal skills, not just with you as parents but also with the students. I'd point out that the first few days at a new school will dominate how a child views the rest of their secondary experience.
I would follow up by saying that I expected a full apology for rudeness and a review of the uniform policy at the school. I'd also be looking at a new school, however, as if you send this letter it will be untenable for your DS to remain there.
It may be worth copying in your letter to OFSTED or the Local Authority, because while schools are being given greater freedoms to do what they want, the more parents who kick up a fuss about this sort of unfair uniform policy, the more likely it is that the government will listen and enforce rules about certain aspects of it.