Trainline

I bought 2 tickets on Trainline app but in a different way I should buy. I realised at the moment I processed my purchase. I contacted immediately trainline customer service and have paid a such expensive amount for this call without get any resolution. I went to the train office in st Pancras station they also could not to do anything and highlight to me avoid to buy anything over trainline. The next day I wrote them an email telling the situation and I got answer back saying I need to send my tickets that I have printed off in a special delivery (the expensive delivery ever) after have paid to call them such money, after I have done that I should wait for about 10 days to get my money back. When I asked them the reason why I should send it in such expensive delivery and why those ticket were so important to them I didn’t get any answer back... I’m really disappointed with trainline service it’s just a shame. Not recommend to no one I already deleted their app and I truly hope no use it back. My question is how can I get legal actions against them? Thank you in advance.