Unacceptable service for Vitality Travel

I booked a trip through Discovery Travel for 16th June 23, from Cape Town to Split in Croatia, Via London. I would be in transit for 1.5 hours in the UK at the same terminal where I landed. I got a Shengan visa and researched whether I would need a UK in transit Visa, as well as called BA to find out. Nowhere did I find any suggestion or instruction that I would need an in-transit visa. I flew to JHB, tried to board my flight to London and was told I could not. My luggage was removed. I was told that I would be refunded. There was 1 other person also denied boarding for the same reason and 17 people the day before. I immediately contacted Vitality to explore my options and cancel my return flight. It was a Friday evening so they were closed. I followed up with 2 emails and have had no response. I had to book another more expensive flight through Amsterdam. Once back in the country, I was able to call Vitality who informed me that I would not be getting a refund as BA had denied this. I believe that Vitality Travel should refund me my ticket, however, I have been denied this.