Terrible and shocking experience; What’s the best way to file a complaint against a Delta employee?

We were flying from Seattle to New Delhi with layover in London. The first flight was operated by Delta and second was Virgin Atlantic. We printed our boarding passes and luggage tags from Delta kiosk and went to the counter for luggage drop.

1. The lady told us we need a UK transit visa. We showed her it’s not required as per policy and we had confirmed from Delta customer support as well. She goes on rambling and gives a phone call to someone. Apparently every Delta employee on checkin counter was on phone call with someone and every checkin was talking at least 15 mins. It seemed they weren’t trained enough to do their job.

2. After a few minutes she says we need to go to Virgin Atlantic counter as this is not a Delta flight. We showed her the boarding pass with Delta name on it and was printed from Delta kiosk. The airline was Delta and our boarding pass was issued from Delta app and kiosk. She rudely said she can’t do anything about it and we must go. At that point we were terribly close to missing our flight.

We went to Virgin Atlantic kiosk and they obviously said it’s a Delta flight and that Delta employees regularly do it because they’re stupid and don’t know how to do their job. A kind lady from Delta helped us finally and we made it to the flight few minutes before the gates closed.

Obviously no transit visa was required and we shouldn’t have been sent to Virgin Atlantic counter.

The stupid employee was a white lady about 50-60 years old, grey hair. I could see she was kinda mean to rest of the employees as well because they were all passing sarcastic/passive aggressive comments to each other.