British Airways denied flights due to transit visas

We recently booked a holiday to europe and booked our flights via LHR to Paris vir British Airways. Our flights were booked 6 months before we travelled. Our Shenghen Visas were approved in 6 days with no trouble or issues. When we arrived at the check in counter to fly, we were denied on our flight as they required a transit visa for a 2 hour layover in LHR. This was not ONCE communicated by the airline. When they denied us checking in, they said it happens every day! If this happens every day, why are you not communicating this small print to your customers!!!! This costed me R 58 000 to get flights literally last minute to try and get as close as possible to our destination. Up to date, I have not even received any notification or mail of our cancellation? This means I cannot claim for any of this. The security told us if we had a Type D Shenghen Visa, they would have accepted it... This was the most horrific experience and also the worst customer service ever. We checked in online and it kept on reminding us about wearing masks but nothing about an additional visa!!! Very ********* to take your money and then deny you when boarding - although you know this happens often with your passengers. I will never support you again and will not be spending my hard earned money on your airline or your country.