It's obviously unfortunate this has happened, but bandying around things like 'consent' and 'professional misconduct' are really OTT...
When you asked to be screened for Edwards and Patau, you've asked them to take your blood and measure it for 3 markers (possibly 4 depending on where you are)
The lab has tested your blood for those, and found the level of each one, which has been reported. You have consented for this test and the levels are presumably printed somewhere on your letter?
Obviously these levels have been compared to the known levels for trisomy pregnancies and normal pregnancies, and you've been given your risk ratio.
Someone has clearly made a mistake and given you the risk ratio for Downs as well as the others, but it is a stretch to say they've 'performed a test without your consent' in doing so, when all they've done is a quick calculation of your HCG levels.
To the PP who said 'imagine opening the letter if you have tested positive for Downs' - it doesn't work like that...
For starters, you get a call and not a letter. Secondly, they will tell you that you've tested as high risk but it isn't conclusive and give you your options for further testing, and thirdly, it is rarely cut and dry at this stage which trisomy it is, so it would be highly unlikely they would be able to promise you they can investigate only Edwards and Patau, and not potentially end up with a diagnosis of Downs. Lastly, if you were carrying a Downs baby and didn't find out with the testing at this stage, it is almost certain that you would find out something was wrong at 20 weeks, when soft markers were found at your anomoly scan. Yes, I know there are some women who don't find out until birth, but those are increasingly rare as scanning technology improves. So unless you are going to refuse all scans, you will ultimately know eventually...
Congratulations on your healthy pregnancy. Surely that's the thing to focus on? In he grand scheme of things, complaining that you've been told your baby is healthy just seems... odd