Discover travel not warning clients of a potential UK Transit Visa

Vacation of a lifetime ruined by Discovery Trave due to lack of information. Why could they not have a bold statement that you need a "UK Transit Visa" if you are South African! In December 2023 we bought five tickets on the Discovery Travel app to fly to Copenhagen on British Airways to the value of R 71 953. I received a discount as I took the whole of 2023 to get to Platinum status on Vitality and I have a Discovery Black account. We app**** for the Schengen Visa we need for Europe but when we went to fly out the airline said we need a Transit Visa for Heathrow although we are not visiting Britain and only transiting the airport to fly on another plane to Copenhagen. So, we had to cancel the flight and book other flights but tickets are now R150 000 at least. The British Airways personnel says this happens often to Discovery Travel clients! Why were we not warned about this Visa? It messed up 18 months of planning this vacation with my family! And the airport attendant said a lot of Discovery clients have the same issue before.