British Airways - Shocking Service

My wife had a ticket to fly ex Johannesburg via London to final destination Rome. Was refused on boarding because she did not have a visa for the UK. She has a Shengen Visa, Passport and only needed to connect onto the next flight in transit through London enroute to Rome. We checked the British regulations and she should be exempted from a UK Visa. She had to buy a new ticket with another airline.....the stress you caused her!!! Now that she is forced to ask for a refund you penalize her with a R1700 cancelation fee. Never have we experienced such shocking service from an airline.....your staff should be trained and advise people correctly on UK regulations. Since when do you need a visa to transit through an airport if it is not your final destination. We fly regularly via Dubai, Frankfurt, Zurich, etc and don't need visas to transit through these airports.