Kiwi.com complaint: They didn’t let us take our flight

Today my 3 family members had a flight from Vancouver to Iran, the airline didn’t let them to take their flight! They said they need visa to change airports in London! But this is not correct, I had checked with the UK embassy 3 times and they told me if you have Canadian visa and your next flight is less than 24 hours you don’t need visa! But in the airport they didn’t understand it! I showed them the UK official website but she said no! She didn’t let them to take their flight for no reason!
They had all requirements for this trip! Now kiwi.com doesn’t accept no responsibility and nor the airlines! This is supper unfair! I lost a couple of thousand dollars very unfairly and now can’t afford to buy them a new ticket!

They said just because this is a self transfer flight! If it was a connected flight we could let you take the flight!