https://contactus.dhsc.gov.uk/Who would you report them to ? I am in same position with Dante Labs for my son's Day 2 test. Have sent 3 email complaints to them and demanded a refund but no response. I've now got my credit card company on the case. Also written to the Which mgazine journalist who wrote the article quoted on BBC website yesterday.
I also had problems with DAM Health for the pre-flight test but thankfully got that resolved after a day of stress. But I got a message from a girl last night about DAM saying they cancelled with them after reading reviews but miraculously DAM then sent them "Negative" results for tests they never had. Like you said it's scandalous and no one in authority is doing anything to stop them.
Plus trading standards for good measure, the highest level of local authority where the company is based.
Dante in Cambridge would be Cambridgeshire CC, but they farm out complaints on trading standards to Citizens Advice. You could report to your local TS, and they would forward it to CCC.