I think we should all expect a bit more than the basics of "doing your job" from HCPs, and I would argue that having a pleasant bedside manner is one of those basics anyway! Having been scanned on a couple of occasions by very rude sonographers (one of whom was informing me my pg wasn't viable at the time!!!), the contrast with having a nice sonographer (and fortunately most of the time they seem lovely) is very striking, and does make the difference between having a reasonable experience and one that is unpleasant or even traumatic. I'm not surprised you are upset.
It was the hospital's mistake, not yours, so whether or not the sonographer was personally to blame she has no right to take it out on you if she had to stay after her shift. I'm sure everyone, whatever their job, has occasionally had to stay behind at the end of their working day to deal with unexpected situations - it's no excuse to be moody.
I know the reason for the anomaly scan is to check for anomalies, but I'm now expecting my third baby, and have never had a sonographer not take their time to get good photos, and try to check the sex (though if the position was such that they couldn't see the sex, then to be fair they can't help that).
I had my 12 week scan done privately this time (due to a mix up with the NHS appointment), at MUMS in Solihull, and they were fantastic. One of the consultants did the scan, and took absolutely all the time needed to explain every bit of the scan to us, answered any questions we asked, etc. It was expensive, but worth the money for the peace of mind. They do a sexing scan there too, and they do let you come back if the baby is not in the right position to see what you're having.