Airline Ts&Cs are not the be all and end all of conditions. If they put anything in there that is found to be in breach of the Unfair Contract Terms Act then it is unlawful and will be set aside in court, plus they will have to pay all the costs. Your local Trading Standards Office will advise you further, and if necessary take it up on your behalf.
In an industry where carriers are openly advertising fares for 1p, no fare can any longer be considered "obviously wrong". Even before that, if they advertise the fare and also take you money for it as full payment, that is generally held in law to be a firm contract. The "obvious mistake" was more to cover advertising something, then saying when you go to buy it that the ad was a mistake.