I have a few questions that would provide a clearer picture:

• What was your originally booked connection time in ATL? Since your flight arrived early in ATL, what was your actual connection time?
• Was your flight departing from ATL nonstop to LHR? (If so, the boarding cut-off is 30 minutes before departure.)
• Have you only claimed reimbursement for items purchased after your bags did not arrive in London?
• Would you agree that U.S. Customs and Border Protection agents are not affiliated with Delta? Other than the U.S. Customs officer lying to you, and being unable to retrieve your bags in ATL, was there anything that Delta lied to you about?

I'm sorry you had a bad experience in a high-stress situation. Long waits at U.S. Customs can happen when traveling with any airline through the U.S. since they aren't employed by or answer to any airline. While choosing a different airline won't help you avoid bad experiences with customs or being rebooked on different airlines, it's good to shop around, so I hope you have better travels in the future.